why there's sex in Erfworld even without the biological significance, as there's no birth nor childhood (POP- there's a new inhabitant fully matured)?

Because it's enjoyable, fun, pleasant (most of the time)!
We have seen that the Erfworlders are very much personalities who, despite not having any biological need for procreation, still want intimacy and companionship. Sex maybe devoid of actual/biological necessity, it's still quite popular. Even the Tool enjoys it enough to spend part of his turn doing it.
the necessity of Sex in Erfworld is social or even psychological. The concept of love is not alien to Erfworld at all and when Love rears it's head, Sex might get involved too. it usually does...

and then there's sex without love, like Wanda distracting the Tool during the dragon raid on ansom's siege....

so, excluding the non-existant biological necessity for sex in Erfworld, it is done for all the other reasons we have in the real world. Pleasure, deep-felt affection for one-another etc.

which makes the erfworlders more "human" or "real" and thus it's sad when they die violently...

Is this comic really a matter of "wargaming chits have feelings too..."?