Core: the central or most important part of something.
Our core is very important for everyday tasks from getting in and out of a vehicle, vacuuming your house to wiping down kitchen counters. It's almost impossible to NOT use your core pretty much in any movement you make.
I hardly ever do strict ab exercises. Like an actual crunch or sit up. But I sure do a lot of compound movements which are basically ab exercises. I like to multitask. Lunges, squats, seated DB exercises, one sided movements, mtn climbers and PLYOS. That's how I work my abs.
ALSO, abs are like any other muscle in your body. Would you train your biceps everyday? No. So don't do sit ups every day. You have to let them recover. Your abs won't grow if you are constantly tearing them down.