I haven't really thought about this question until my receiving some special custom orders recently. And I "googled" how men are actually thinking about and agree with general aspects on the belt. There are possibly two main reasons to summarise: 1. For the final touch - belts can complete your look for the day; 2. For practical use - we need belts to hold our trousers where they should be throughout the day.
While writing this post, I can't help adding one more : For your collection - you can treasure belts if they mean so much to you for some special reason and you might want to add them into your private collections.
I've been asked to work on men's belts so many times and now finally decided to add some belts to my small collections. Let's fingers crossed how they are turned out later. But for now, I am very happy to send one off first.