It *is* a big topic. There was some discussion in the controversial section a long time ago about it. Having had boxers my entire adult life, I've seen some pretty ugly conversations about this topic. As Joao said, it used to be done for working dogs to prevent injury to a very susceptible part of the body. Now, unless you have a working dog, it's mainly done for cosmetic reasons. IMO.
I've had both cropped and natural dogs and I truly didn't notice that the natural eared dogs had more problems with infections. That mostly seemed to depend on whether they suffered from allergies and whether I took care to dry the ears after swiming, etc. Hematomas? A huge problem with my natural eared boxers, just like happy tail with the natural tailed dog I fostered. But again, those boxers tended to have allergies and shook/scratched their ears a lot causing injury.
The boxer show crop is long and elegant. It can be difficult to find someone that can do it properly and then it must be taped to stand. Sometimes posting and taping has to be done for extended times. It's a lot of work. Some say that they have had cropped boxers that only required massage, but I have yet to see any pictures of a show crop that didn't have to be posted and taped to stand properly. Because natural eared dogs have virtually no chance of doing well in the show ring, ear set was not something than most breeders worked on in boxers. European dogs usually have wonderful ear sets, while many American lines do not. It's a shame, really, because a natural eared dog with a good ear set is beautiful.
I have no problem with cropping as long as it's done by a qualified veterinarian in the proper time frame. That means no cropping much after 12 weeks and doing so only under anesthesia. I really do love the look of a cropped boxer. It completely changes their look. Here's a link that shows proper ear set.
Ears
And here are some real life examples from my house. Just because pics are fun.
This is a bad ear set. Roy has his mother's ears. I was worried about that when I saw her and no amount of massage or taping helped. Had he been cropped, his ears probably would have stood wonderfully.
Roy and Al, two handsome boys. Both with junky ears.
And here is Leo with a show crop. No hematomas for him, but I did have to watch his ears and dry them well after a bath.
And here is Vincent Van Gogh with his scissor cropped ears. No excuse for this.
Btw, that's my Ed on the left. He had good natural ears.