A great many people -- including professional "chefs" -- do not know how or when to use a slicer. That doesn't make a slicer a "gimmick." Slicers serve many purposes.
A chef's knife can do all of the same things a slicer of similar length can do. But because a slicer is narrower than a chef's, it doesn't stick in the cut as much a chef's.
A long slicer, 240mm or longer, allows you to use long, draw strokes; that means less "sawing;" which is valuable because sawing leads to ragged surfaces and single cuts leave smooth surfaces. A long slicer also makes for an elegant and efficient carving knife. An attractive slicer is "eye-candy" at formal dinners, whether at home or in a commercial setting. So are the knife skills and sharp edge required to wield it properly.
A slicer shares a profile with the sort of paring knife the French refer to as couteau office. Petty knives, which are short slicers, 125mm - 210mm, are versatile enough to pare, bone, tourne, or just about any other short or medium length knife task.
While I write about a slicer as one of the four "essential" knives in a kit which is both bare bones but sufficiently well furnished to do everything well, it's good to remember that I'm drawing from my own experience. Just in case it isn't glaringly obvious, I don't know everything about knives.
Profiles are far less important than sharpness. Just about any sharp knife can perform just about any knife task better than just about any dull knife. But with reasonable skills, profiles can make quite a bit of difference. The knife skills necessary to make good cuts are VERY easy to develop. While they take a willingness to learn, a little bit of time and some patience, you don't need to be professional.