I noted that if I downvote an answer I lose one point. However I don't understand the reason of this. If I downvote an answer which is wrong, I help the other reader to understand which answers are correct. Can someone explain me this fact?
Receiving a downvote is not nice and they should thus not be cast carelessly. As a disincentive against careless or even vindictive downvotes, one had to invest a point to downvote a post (unless it is in CW mode and points are moot).
It is true that this can lead to legitimate votes not being cast, and this is a problem, too. Indeed, at some point it was decided that for question post this point is waived, so that now it is only for votes on answer posts that there is a point to be invested.
In the end it is a trade-off. The current balance works about alright.
Finally, if there is an actual error in a particular answer it will make sense to explain it. The down-vote in addition then becomes not that crucial. This is slightly different for questions, which can be considered as a reason for the different handling.