Do you know why they do that?
Is it for flavour? No.
It's to stop flies and clean the neck.
Bad image really.
You push it in your drink, get one of those at the bar - they will put it in the glass (can not say pint as you do not get pints here in most bars etc) and if you have the bottle it will be in.
By Case of that it will often come with lemons (most common to put in) or lime (most prefer lemons though)
Part and parcel of that beer, it is not for decoration
So is it for flavour? Actually YES!