What is the purpose of animals and plants on Earth?

It should be noted that Allah has created man to worship Him, as He says (interpretation of the meaning): 

“And I (Allah) created not the jinn and mankind except that they should worship Me (Alone)”

[al-Dhaariyaat 51:56]

Allah has created for man and has subjugated to him what is in the heavens and on earth, so that he may benefit from it and enjoy it, and meet his needs, and use it to help him to worship Allah.

Allah says (interpretation of the meaning): 

“He it is Who created for you all that is on earth”

[al-Baqarah 2:29] 

“See you not (O men) that Allah has subjected for you whatsoever is in the heavens and whatsoever is in the earth, and has completed and perfected His Graces upon you, (both) apparent (i.e. Islamic Monotheism, and the lawful pleasures of this world, including health, good looks) and hidden [i.e. one’s faith in Allah (of Islamic Monotheism), knowledge, wisdom, guidance for doing righteous deeds, and also the pleasures and delights of the Hereafter in Paradise]?”

[Luqmaan 31:20]

“And has subjected to you all that is in the heavens and all that is in the earth; it is all as a favour and kindness from Him. Verily, in it are signs for a people who think deeply”

[al-Jaathiyah 45:13] 

“And He has made the sun and the moon, both constantly pursuing their courses, to be of service to you; and He has made the night and the day, to be of service to you”

[Ibraaheem 14:33]

There follow some of the reasons and purposes for the existence of animals and plants on Earth: 

Plants are food for people and animals. Allah says (interpretation of the meaning): 

“Verily, the likeness of (this) worldly life is as the water (rain) which We send down from the sky; so by it arises the intermingled produce of the earth of which men and cattle eat”

[Yoonus 10:24]

“Then let man look at his food:

We pour forth water in abundance.

And We split the earth in clefts.

And We cause therein the grain to grow,

And grapes and clover plants (i.e. green fodder for the cattle),

And olives and date palms,

And gardens dense with many trees,

And fruits and herbage.

(To be) a provision and benefit for you and your cattle.”

[‘Abasa 80:24-32]

Plants remind man that this world will come to an end: 

Allah says (interpretation of the meaning): 

“Verily, the likeness of (this) worldly life is as the water (rain) which We send down from the sky; so by it arises the intermingled produce of the earth of which men and cattle eat: until when the earth is clad in its adornments and is beautified, and its people think that they have all the powers of disposal over it, Our Command reaches it by night or by day and We make it like a clean-mown harvest, as if it had not flourished yesterday! Thus do We explain the Ayaat in detail for the people who reflect”

[Yoonus 10:24]

Allah gives people the likeness of the resurrection of the dead and their coming forth from their graves, by reminding them of what they see of the earth coming back to life after it was dead. He says (interpretation of the meaning): 

“And among His Signs (in this), that you see the earth barren, but when We send down water (rain) to it, it is stirred to life and growth (of vegetations). Verily, He Who gives it life, surely is Able to give life to the dead (on the Day of Resurrection). Indeed He is Able to do all things”

[Fussilat 41:39]

Man also derives medicines from plants, the greatest of which are the black seed of which the Prophet (peace and blessings of Allah be upon him) said that it is a cure for every disease apart from death (narrated by al-Bukhaari, 5687); and honey, which is gathered from the nectar of flowers, of which Allah says (interpretation of the meaning): “… wherein is healing for man…” [al-Nahl 16:69]. 

Animals are also food for people. 

Allah says (interpretation of the meaning): 

“And the cattle, He has created them for you; in them there is warmth (warm clothing), and numerous benefits, and of them you eat”

[al-Nahl 16:5] 

“And He it is Who has subjected the sea (to you), that you eat thereof fresh tender meat (i.e. fish), and that you bring forth out of it ornaments to wear. And you see the ships ploughing through it, that you may seek (thus) of His Bounty (by transporting the goods from place to place) and that you may be grateful”

[al-Nahl 16:14]

And animals are a means of transportation and an adornment.  

Allah says (interpretation of the meaning): 

“And (He has created) horses, mules and donkeys, for you to ride and as an adornment. And He creates (other) things of which you have no knowledge”

[al-Nahl 16:8]

From animals man makes the tents in which he lives and the clothes that he wears. Allah says (interpretation of the meaning): 

“And Allah has made for you in your homes an abode, and made for you out of the hides of the cattle (tents for) dwelling, which you find so light (and handy) when you travel and when you stay (in your travels); and of their wool, fur, and hair (sheep wool, camel fur, and goat hair), furnishings and articles of convenience (e.g. carpets, blankets), comfort for a while”

[al-Nahl 16:80]

Horses are a means of jihad and will remain so until the Hour begins; the Muslims wage jihad on horseback and achieve victory, acquire war booty and are rewarded by Allah. 

Allah says (interpretation of the meaning): 

“And make ready against them all you can of power, including steeds of war  to threaten the enemy of Allah and your enemy, and others besides whom, you may not know but whom Allah does know”

[al-Anfaal 8:60]

Al-Bukhaari (2852) and Muslim (1873) narrated from ‘Urwah al-Baariqi that the Prophet (peace and blessings of Allah be upon him) said: “Good will remain (as a permanent quality) in the foreheads of horses (for Jihad) till the Day of Resurrection, for they bring about reward (from Allah) and booty.” 

These are some of the benefits of animals and plants. And there are other purposes and benefits which no one knows except Allah. 

And Allah knows best.