>>1957058It's a bit hard to explain if you aren't used to roundabouts but the issue is if you're not indicating when you enter a roundabout then people will assume you're going straight through it, and so if someone is coming from the opposite direction they'll enter it even when you're in there because they know you aren't going to cross their path.
But if you are in fact going to take the third exit/ turn left (in a right hand side drive country, like this diagram) then you need to indicate left, so purple would be doing this and blue would know not to enter the roundabout if purple is already on it. But if purple wasn't indicating and was on the roundabout blue would enter thinking that purple is going to go straight through and they won't cross paths, do you follow?
There's lots of small single lane roundabouts where I live where people drive according to this logic