A car approaching a roundabout has no options to communicate to other motorists with a signal as I understand it.
(I am using north american driving conventions below)
The primary purpose of signalling as you enter is to tell drivers already in the roundabout if you intend to:
Single lane roundabout:
a) merge into the roundabout (left signal), or b) glance the corner and make an immediate right turn (right signal).
Two lane roundabout:
a) merge into the roundabout - the inner circle, or b) stay in the exiting lane (outer circle) and immediately take the next right turn.
In the single lane roundabout's case, it's possible to make a right turn from a two-lane approach without getting in the way of traffic already in the circle. People in the roundabout will know you are not about to cut them off. You can do this exiting from Square One - if you're making a right, you don't have to stop for people in the circle unless they are exiting there as well (and this is why conversely, you need to signal right before your exit). Your signal shows your intention.
In the case of a two lane roundabout, it lets other drivers know if you are merging into the inner lane to travel around (left signal), or staying in the outer exiting lane (to take the immediate next right - right signal).
Interestingly, people in Mississauga often make errors with this type of signalling in dedicated right turn lanes at traffic lights with yield signs. You should signal to the left here to show you want to
merge (most people signal to the right).
In mini roundabouts, it additionally serves the same function as signalling at a normal 4-way intersection. The circle is so small that you can clearly see all approaching sides. People drive almost straight through the circle with these (its just a big dot painted in the centre of a 4 way intersection).
In short, the purpose of signalling
before you enter is to let people already in the circle know if you're going to cut them off or not. The purpose of signalling
before you exit is to let people waiting to enter know that you going to be leaving.
As you approach, you're not literally signalling to show where you will eventually leave, you are signalling to show if you are merging into the circle or not.
*with a single lane, single approach roundabout, this is less relevant. It just lets the people at the entrance ahead of you know if you're going to be taking the immediate next right (so they are clear to enter knowing they aren't going to get in your way).