Removable, washable pads are one of those features you don't appreciate until you've ridden without them. They attach with Velcro so you can pull them out and hand-wash them after sweaty rides, which keeps your helmet fresher and helps prevent the skin irritation and breakouts that can come from bacteria buildup in non-washable padding.
Beyond hygiene, removable pads give you real fit flexibility. Many helmets come with different thickness pads so you can fine-tune the fit around your head shape, and you can swap in fresh pads when the originals wear out—which happens faster than you'd think. Replacement pads are usually inexpensive and extend the life of your helmet considerably.
If you ride more than a couple times a week, especially in warm weather, removable pads are absolutely worth seeking out. Budget helmets with glued-in foam pads work fine for occasional use, but they get grimy fast and can't be refreshed.