can anybody tell me what the other planets in our solar system are actually for?

My guess is to explore them.

Your statement that "everything in nature has a purpose" is incorrect - wildly so. Absurdly so. On Earth, things seem to have a purpose because the whole ecosystem co-evolved. Everything had to fit together, or it would have ceased to be. Things that did not fit their ecological niche died out long ago, leaving only those things which "worked". Off of Earth, nothing (excluding of course the sun and moon) can really be said to have a purpose until we humans give it one. The other planets will no doubt be explored one day, and then exploited. It's what we do.

Nothing in nature has a purpose - it just IS. On the other hand, if you ask why, the answer is that that is how the dynamics of the solar nebula worked out. Small anomalies in the distribution of matter in the nebula became bigger anomalies under the influence of gravity. In the outer solar system, these eventually formed the gas giants, In the inner solar system, where temperatures are higher, volatile components tended to be lost to space.

They have the same purpose as any rock on the ground. They just are.

Your additional details are irrelevant. You confuse cause and effect We are here because of those things, they are not here for us.

The outer planets are deflectors for comets & asteroids that would otherwise use Earth as a target.

Methinks you should ask Shroedinger's cat.

There is no purpose, life is explained by evolution.