Should I use a class even for a single html element?

I am working on a website design and my boss told me that usually ids are never used and classes are preferred, but he doesn't remember why. So I did some research and I found this this, but the first answer does not explain very well why ids should not be used. I thought that for an element that is used only once ids were used. If I won't use an element more than once, why should I use a class?