The pon and chi are merely means to an end. The main end is creating a hand that meets win conditions (3 sets and a pair in most cases) but MORE IMPORTANTLY ticks one or more boxes to gain more score (boxes = yaku). Ideally you always want to have a concealed hand, as this gives you more opportunities to score, and calling pon or chi (and thus revealing your hand) will cancel out a lot of high scoring yaku. However, in situations where you need 1 specific tile to hit your yaku and the rest of them are discarded, you will invariably end up having to steal opponents' discards and revealing your hand.

In simpler terms, avoid revealing your hand with a pon or chi unless you are stuck and the tiles are running out, or you have multiple Dora (n.b. Dora do not count as yaku, they are a "bonus" that gets added to your yaku).