What is our purpose here on earth?

[robvalue]

I'd say:

Reproduction has a purpose from the point of view of the survival of the species.

Sex organs have a purpose from the point of view of reproducing.

A key has a purpose from the point of view of opening a door with a corresponding lock.

None of those things have any purpose from a neutral point of view, or from the point of view of analysing minerals deposits on Mars. (Except indirectly by keeping us alive longer to do it).

To say something must be viewed from the point of view that gives it a purpose is to beg the question. Anything could have a purpose, to someone or something, somewhere. "Intended" purpose is perhaps a better phrase. This then applies to the key, but not to reproduction. The key was intended to open a lock, but reproduction was never designed and so wasn't intended to do anything.