Great sort of question.

The answer would likely vary based on type and location of planet. As someone already pointed out, Jupiter is our goalie, keeping out the pot shots against Terra. Saturn is a host for life by way of its moons (possibly). Others may just be storage of chemicals and such so when Sol goes boom, some of that is carried away to eventually make more planets and life... cold storage as such.

Heres a thought. just by asking this question you give meaning to lifeless planets existence. Maybe they are there just so we can ask this question and thus further or deep thinking.

What is the purpose of a lifeless planet? None. Unless... you answer yes to this question. Is reality a product of intelligent design? If so then even lifeless planets out in the void may serve some purpose. You'll have to ask the creator about that one.

to show earth is not some kind of fluke or a random occurrence.

if the definition of evolution is the adaptation of life to it's surroundings then their is no reason the other planets should not have life.

You're only asking that question because everything seems so big to us. What if everything in the observable universe is actually the size of a small fish tank to some being or entity (God?) outside of our dimension? The universe obviously wasn't designed on the human scale. So it's logical to surmise that there may be beings for whom Jupiter, or even the sun, is what a grain of sand is to us. We certainly don't ask what's the purpose of all the sand on the beach. And on a certain scale, grains of sand are going to be as varied, unique, interesting and even as massive as the planets and stars are to us.