is there no difference between there and to there (like in Russian: tam/tuda)
"To there" doesn't exist in English, as far as I know. :) Czech also doesn't tell the difference. It does it only with "tady/sem".
There really is no need to. Just like in English. It is clear from the rest of the sentence if something is THERE or if we are heading THERE
I get why we would say "Proč tam chceš jet?" but isn't "kam" the word for "to there"? Would we say "Kam jede tato ulice"?
No, "kam" does not mean "to there" but rather "to where". So:
where = kde
to where = kam
here = tady
to here = sem
there = tam
to there = tam
Of the above, it is only "tam" that applies both to place and to direction toward a place.