I've been happy with Blizz' default UI, is there any benefit other UI's give other than just looks, do they let you do things easier, for example?
Yes they can improve your level of play by either more clearly displaying information or displaying information that otherwise wouldn't have been shown at all.
As an example, from mine posted above for instance, I can see above my own unit frame the buffs I have given myself during combat which are very important for me to maximize uptime of. These are displayed as part of the default blizzard as well as mine in the top corner, however in a raid I would have so many buffs that picking 3-4 from a wall of 30~ to monitor their time remaining to reapply within the last second would just be near impossible.
But at the end of the day, UIs looking nice is one thing, their actual function is to convey information ... so simplifying your UI to show only the exact information you require as well showing it clearly where needed will be a big boost to your performance.