To taunt the earth. Belittle the earth.
Their like mile markers on a highway
They are there only to protect Earth from big asteroids. The more planets, the more likely they will take one for good 'ol planet Earth. Other than that, they have no purpose.
Just keeping us company.
To keep astronomers busy .
You don't remember your science classes do you?
IMO most of them were inhabited in the past. So they did serve a purpose back then to the beings who inhabited them :) . Unfortunately we may all suffer the fate these other planets did in the past someday.
Those are inertial or gravitronic cyclic extenders. Like the sets of gears in a clock mechanism that makes a twenty rotation main spring create thousands of rotations. At a macro scale of course.
catching the flying rocks and trash
To make sure my ice cream cone doesn't drip all over me...
To keep us wondering