
Medically, castration prevents testicular cancer. That's it. Behaviourally, it may help some hormonal behaviours (e.g. I've recommended a client's dog be neutered because he is absolutely obsessed with scenting other dogs to the point of licking up wee, and I cannot walk him with others as he spends the entire walk trying to get to them to sniff their bits no matter how many times he's met them). But, it may make some worse, most notably if the dog is at all nervous or prone to being spooky, because you're taking away that hormonal support.
In that respect, as Brainless has seen, things like humping and marking can become much worse because there are, in many cases, driven by insecurity. And certainly aggression, which is a fear-based behaviour in the vast majority of cases.
On the whole, if the dog is happy and healthy then I would leave well alone.