There is no polite way to say this: males can look amazing or have big balls, but not both....
Ornamentation, such as big manes, long beards, swellings and brightly coloured skin (think of the red bum of a dominant male mandrill) serve to impress females and intimidate other males.

Big testes, relative to body size, roughly correlate with high levels of sperm production, which increases the chances of successful fertilisation during copulation.

Either trait is appealing, but it seems evolution forces males from all species to choose between one or the other.

“Ornament elaboration comes at the expense of testicle size and sperm production,” says Lüpold. “In a nutshell, the showiest males have the smallest testes.”
Something, something, Donald Trump.