What is the purpose of planets on our solar system?

Whose purpose?  Phoebe? (Greek goddess, Titan, identified with the moon)... Zeus?  Ra?  Abrahamic God?  Nature?  The Universe? For each of these there would be a different, totally speculative, answer. In the case of Nature, or The Universe, the evidence is strong that neither of these has any "purpose".  They just ARE, and any effects observed are the result of physical laws playing out, sometimes creating what seems to be "benefits" (helpful tides, light at night), sometimes "disaster" (supernovas, plague), and sometimes neutral phenomena (the void of intergalactic space, this small rock I hold in my hand, the death of an earthworm.) On this basis I dispute your assertion that "We know that everything has a purpose..."

How do we know this?

To scientists the word "purpose" is a problematic word.  It often comes with philosophical baggage that has no scientific basis.   The planets do no have a purpose.  The planets are consequences of physical law. However, if we agree to talk of "purpose" as being a narcissistic human-centered outlook,  then we can speak of this kind of purpose.  Man makes use of X and flips it in his mind to say that X exists for that use (has a purpose of being useful).  One can make a variant on that old Carly Simon song, "You're so vain, you probably think this universe is about you." ;-) But with that outlook, which certainly has existed for most of recorded history, the perspective is: Earth is home, the center of the world.  We see the heavens unfold around us like a dome over our heads. Sun is light, warmth, a calendar for telling the seasons Moon is light in the night, cause of tides, a calendar for telling the months Other planets, "wanderers" in the sky, associated with pagan gods, their positions used to cast horoscopes. However, this changed with the work of scientists like Copernicus, Kepler and Galileo.  Copernicus argued that the sun, not the earth, was the center of the solar system. Galileo found moons revolving around Jupiter.   We were no longer at the center of the universe.  Centuries later other scientists, showed that we're just one planet around one star in one galaxy among many in a universe that has no center. So seen in that way, the "purpose" of the other planets is to give us a broader outlook on our position in the universe, to show that our position in it is not privileged.

The exo planets perform a key role in our survival. Jupiter and saturn are massive and deviate any kind of comet or asteroid that comes our way. They act as sweepers so that any interspatial debris will be trapped in their lagrangian points L4 and L5. Jupiter saved us from the shoemaker levy comet. Another purpose is to use the gravitational slingshot effect when space probes are sent far away. They gain a lot of energy because of slingshot effect and this was used in new horizons mission to pluto. The inner planets sometimes trap comets which may affect us.

Purpose is guess is the localized term. Leave aside these heavenly bodies. For us we may think even a Mosquito or a House fly don't have any purpose. They just bring diseases for us humans. And probably mosquito will think the same that these giant humans don't have any purpose they just kill us and they don't even eat us. Similarly we just may be a spec of dust in our milky way and looking from that perspective even we and or our earth doesn't have any significant purpose. Now coming specifically to your question, if we look at scale of our solar system. like you said moon is controlling the tides and earth is harboring the life. Same credits must go to all of the planets. Our position in solar system (size of earth and distance from sun etc.) is due to the perfect balance of gravity and distance of all the planets around which keeps our atmosphere intact and enough heat to keep oceans flowing. Otherwise there will be all ice or just no water at all. Even of one of those planets is misplaced rest of the planets have to realign themselves due  balance the gravity. And we may have to shift our orbit nearer to sun which will burn us or away from sun which will freeze us. Or who know we may collide with our moon or any other planet while doing so. So in my view its not the purpose which defines our position. Its the position which defines our purpose.

This is a question that has haunted me for years, and still no satisfactory answer.  I earlier thought that each planet had some relation to another, but now I cannot reason out why.  Ergo, I feel that there is not one purpose other than Earth to sustain various forms of life.