Convertible snow pants give you the best of both worlds — they're standard waist-height pants that come with removable suspenders, so you can attach them when you want extra security and take them off when you don't. The suspenders keep your pants from sagging or shifting, which is especially nice on deep days or when you're hiking terrain, but they don't provide the full chest coverage that a true bib does.
These are a great pick if you ride in variable conditions or split your time between resorts and the backcountry. On cold, deep days, clip in the suspenders for a more secure feel. On warm spring laps, ditch them and ride lighter. They're also ideal if you're on the fence about bibs and want to test the waters before committing.
The tradeoff is that you don't get the full snow seal of a bib — snow can still sneak in at your waist during a tumble. And there's a small added hassle of dealing with the suspender hardware. But for riders who want versatility without owning two separate pairs, convertibles are a smart middle ground.