Weighted sit-ups are helpful to increase strength and size of the abdominal muscles because they add stimulation that you can't get without using weight.
Weighted Incline Sit Up
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How to Build Impressive Abs with the Weighted Sit Up
I did weighted sit-ups every day for a week — here's what happened
20 Ab Exercises With Weights for a Shredded Six-Pack - Men's Journal
What are the benefits of doing sit-ups and crunches with weights while keeping your feet on the ground? What are the benefits of doing them in midair (leg raises)? - Quora
4 Benefits Of Sit-Ups + Why Your Next Workout Routine Needs Them