One simple use-case is shortening if statements a bunch.
For example, if you have a character that contains a mish-mash of Unions, Meshes and Parts, and you have to change all their collision groups, you have 2 ways of writing it.
1st way, not good:
if part:IsA("Part") or part:IsA("UnionOperation") or part:IsA("MeshPart") then
2nd way, cleaner:
if part:IsA("BasePart") then
As I said, if you’re looking for a specific part type, search for the specific part type. Else, you can use BasePart.