You're indeed signaling that you are about to change lane or exit. People waiting to get in know that they can do so if you're in the outside lane, or that to the contrary they should not take the outside lane if you're inside.
Entering: No, that's pointless. Of course I'm going to turn as I enter the roundabout. What's my alternative? Entering the wrong way? Driving straight across the island?
Exiting: No, that's pointless. Signal all you want, I am going to wait 2 more seconds to enter the roundabout until I see if you are actually going to take the exit.
If you are on the inside lane of the roundabout and prepare to exit, you signal to warn people already on the roundabout, and tell people waiting to get in that they shouldn't get on the outside lane even if it's free.
If you are on the outside lane and prepare to exit, you signal when the road you'll take is next. Whether you'd personally wait is irrelevant; changing lanes and direction always warrants a signal, roundabout or not.
