The question says it all, so if you know the answer to this, please help.
ToObjectSpace and to world space is commonly with Motor6Ds to manipulate the C0 and C1 of the joints as they are relative the the part0 object space and part 1 object space respectively.
Same goes for the new bone instances with skinned meshes
Here is an example with point to object space used for a Motor6D turret
And also welds can also have C0 and C1 properties and hence be CFrame animated but usually we expect welds to not move like Motor6Ds so we use Motor6Ds instead of welds.
There are probably other cases but personally I mostly work with Motor6Ds CFrame animations where ToObjectSpace is used a lot so that should show the possibilities of using the CFrame functions.