There are a number of ab exercises show up in everyone's workouts when I train them. This is another one:
I've written many posts on the plank and its variations.
These include the normal plank and its variations, and the side plank and many of its variations.
Why I use them.
When you're moving heavy objects (in or out of the gym), playing sports, or resisting sudden force from a fall, you need strong abs to stabilize your body. Planks and the variations of them are excellent for directly training that stability.
I do "cheat" a little - if a client is doing pushups, I don't have them do planks. I still count this as a plank inclusion - a pushup is just a plank with a push, so clients generally don't need both the same session.
Conversely, if a client is struggling to do pushups and his or her hips are sagging down, I'll add in extra planks on top of the pushups. They need midline stability, so I'll put some extra effort on it.
How I program them.
I try to work up to up to 30 second holds per side on side planks before I progress them to more difficult variations. I try to work planks up to 60 seconds consecutive. Occasionally I'll go as much as 2-5 minutes, if midline endurance is an issue for the client.
When these are difficult, I'll break it up. 30 total seconds on each side. 60 total seconds of planks. Once you get one full 30 or 60 second set, I'll start adding sets. I'll try 2-3 sets, usually between other exercises (simply to save time.)
These planks can be surprisingly difficult. You won't "feel your abs" working them, generally, but you will be fatigued more than you might suspect. As always, if you choose to include these in a workout, start slow and work up. Don't just try and push until you feel it - start with a little and add more each time.