Anyone with a computer with no other interface options needs to get a mouse since, otherwise, they won’t be able to do much other than turn it on and off. However, there are also some scenarios where getting a mouse would be worthwhile, even if your setup already has a cursor interface.
For example, pretty much all laptops these days use touchpads. These are useful, but depending on what you’re doing, you may prefer a mouse’s precision or comfort (or even just the familiarity). In some cases, it may also be worth having more than one mouse available for different tasks (i.e., one for work and one for gaming).