Responding to a simple overcall (SAYC)
There's a few reasons to take a bid here.

The bidding has gone 1C-1S-P. This indicates that RHO likely has fewer than 6 HCP. Let's consider a few different scenarios.


If partner has only 8 HCP, LHO has an extremely strong hand. You would like to force LHO to take a second bid at 2NT or above, rather than at the one level. 2SX is likely to be fairly lucrative -- you will most likely take 5-7 tricks in trumps alone (partner won't open on 8 HCP missing both top spade honors, and your 9 card fit suggests multiple ruffs are possible), while opponents likely have a game, which they will find if you allow the inexpensive 2-level call.
If partner has 12-14 HCP and balanced shape, 2S is likely to make and nobody is likely to take another bid; if you pass 1S you'll likely allow opponents to find a making 1NT or 3C, or even a making 4H if opponents have a good fit and LHO is allowed to make a 2H call (even worse, partner may double a making 4H if you don't warn that their spade honors likely aren't cashing).
If partner has 15-16 HCP and 6-3-2-2 shape you may find yourselves in a making 4S, but only if you speak up now.


You don't give much detail about your hand, but with a known 9+-card fit and weak values you should consider Tom Au's suggestion of making a 3S call with a hand that suggests it will be the right thing to do. A singleton or a void with few HCPs suggests this, especially if the shortness is in clubs. A hand that is likely to take fewer than 2 tricks on defense should also consider 3S.