Stamina work like kegels are vital! You should be doing them regularly even if you aren't looking to enlarge your penis.
The PC (pubococcygeal) muscle aids in urinary flow and contracts during an orgasm while the BC (bulbocavernosus/bulbospongiosus) muscles aids the erectile process and contributes to orgasm and erections. Both sets pof muscles should be worked for maximum benefit.
Here's some details and diagrams of both sets of muscles:
Pubococcygeus muscle - Wikipedia, the free encyclopedia Bulbospongiosus muscle - Wikipedia, the free encyclopedia