In my natural state, you would most often find me in desperate need of a haircut, a shave and a wardrobe consisting of something other than black T-shirts with skulls on them. Still, there are some things I'll dress up for. Theater, for example. You can wear jeans to the cinema, but at the theater, there are live human beings extending you the courtesy of performing for you, and this you need to respect - regardless of whether the performance is good or not. I won't dress formal for a burial, on the assumption that the key figure really couldn't care less any more, but I do wear a suit and tie when I go to see a play.
Every man needs to own a formal suit, even if he will only use it twice, to be married and buried. And every man needs to know how to tie a double Windsor, even if on the second of the abovementioned occasions somebody will do it for him.