Hard to explain without really experiencing it yourself. Essentially, pretend it was possible to have a "useable" socket with over 100 "sides" it would obviously be practically a circle internally. i.e. There are so many "sides" they are hard to even make out as sides and it seems internally almost like a smooth circular socket. This would obviously provide poor grip on the corner pieces or each side of the nut you were trying to grip (the bits it comes into contact with) as such a small area of the nut and socket are touching/gripping.
In such situations as this, even with a 12 sided socket, sometimes on stubborn nuts/bolts the socket can literally slip and keep turning past the edges/corners of the sides of the nut/bolt or whatever you are turning.
If you can imagine the other simple extreme. Say you had a 6 sided nut, and a 6 sided socket both 10mm. There is no better alternative. The socket perfectly aligns its sides to the sides of the nut and maximizes its contact/grip/friction on it as shape to shape, it aligns and is a very strong "bond" if you like.
Or another analogy. Imagine trying to undo a triangular shaped nut with a "normal" hexagonal socket. Would be pretty poor as would only grip at the 3 extreme corners of the triangle assuming you got the right sized socket. Best to get a proper triangular shaped socket.
Hope I explained that well. Not to say that those types of sockets ar ebad, they are convenient as they maximize the chance of finding a socket that fits odd sizes all be it not always perfectly.