In many places it is said to "Serve risotto on a hot plate - or else" or "you better serve it on a hot plate".
Why is that so?
What moscafj says is true, but it doesn't explain specifically for risotto.
Basically, the creaminess** is because of a starch-based solution. Starch based solutions are temperature dependent and will get much thicker as it cools. If there's enough starch, it can solidify to the point where the sauce will break into pieces if you try to manipulate it.
** Don't start with the 'it's not creamy because there isn't cream in it' crap again.