As a few have said above, training your abs is important to developing overall strength, as well and helping you to move more weight, safely, on the big compounds - core strength essential on squat and deadlifts in particular, along with anything overhead.
Sit ups and crunches are not the best though (I’m not one to hate on any exercise as most have a place but they give limited bang for your buck).
Anti extension or rotation has been shown to be hugely effective along with isometric holds.....so, ab roll outs (seem to recall this being scientifically proven as providing most ab activation), paloff press (personal favourite), fall outs, anti press, waiters/farmers walks/carries, planks (all variations and hollow holds (I do this Tabata style for 4 mins - brutal but effective) are all really good.