Starites are things you need to collect to complete the level.
Starites are the things Maxwell needs for an unknown reason.
They are affected by physics.
They can not be grabbed by NPCs.
As seen in super scribblenauts, starites seem to make up constellations, which may be why maxwell collects them. But, it is unknown why maxwell and other scribblenauts need to make constellations.
In Scribblenauts Unlimited, Maxwell needs starites to save Lily from a curse, but after he saves her, it is assumed he still collects them.