if you are keeping two males together, it is not necessary as such.
It is mainly to keep your male from getting females pregnant if you have some. As baby-wilson said, it doesnt change their character.
They will still fight over girls, they will still try to hump girls, its only to keep them for getting sows pregnant.
As you only have boars, you wouldnt have to do it.
However, as boars get older they sometimes dont get along as well as they did, in some cases they have to be seperated.
The older a guinea pig gets, the more difficult it is to introduce them to new ones, especially with males.
As the putting together of males and females has proven to be easier in 90 percent of the cases, alot of people neuter their males,
so that in case of a fight, they can just seperate the males and put each of one with some girls, and as they are already neutered, they wont have to sit alone for three weeks.
Also think about the natural course of things - two male piggies, getting older, one dies.
It would get difficult (not impossible, but very difficult) to introduce a young boar to that old one.
Girls would be easier - but it might be that at this point, your old boar is too old to be castrated, as with age ANY operation gets more risky. So again, people have piggies neutered to be on the safe side, sort of like an investment in the piggies future happiness.
To make sure that whatever happens, the piggy has ALL options open, and doesnt need to be alone.