There are many times when you need to scale an irregular-shaped mesh up or down, but the scale tool doesn't give the right results. Take the letter e: create two 3D es, and try and scale one down to fit inside the larger shape. You just can't do it - somewhere there will be an incorrect overlap.
slide solves this problem, because it works at the point level, "sliding" vertices along their normal, or "the way they point". In contrast, the scale tool always scales from the axis of the object.
Two quick ways of modeling form-fitting clothing: Model your body mesh, then copy it. Chop off the arms and legs (select those vertices and use the Delete Vertices tool), then slide the points out slightly. This new object is your clothing. Or instead of copying the mesh, just use slide's lasso to select the areas of the body mesh that will slide out to become clothing.