Hi yes as a guy I have done them regularly since I was a teen. You don't need to be erect to do them but initially it helps you understand and focus on the correct muscles your trying to contract if you can see your erect penis moving up and down hands free. Another is to stand when peeing and keep stopping your pee. Once you've got the core of these muscles then you can grip or flex them anytime . I find sitting or driving is a good position.
Why do guys need to do them ?
Actually there are a few good reasons. One is blood flow , to maintain a good firm erection then blood flow is Paramount. Doing these exercises will help maintain even increase it.
Next is if you understand the male internal sexuual anatomy then the PC or pelvic floor muscle can clamp on and bear around the prostate and Cowper's glands. Enabling you to both stimulate your own orgasm and enhance it but also with control and practice hold back your impending ejaculation. I could go into more depth but if your interested its explained and taught in the Kamasutra and tantric methods.
Finally if both you and your partner have practiced and built up muscle tone its possible to have sex by purely using this exercise and no pelivic thrusting required at all. For the woman its known as the Shanghai grip supposedly first used and taught by Geishas in its brothels. The gripping and squeezing is like milking the penis.( this is rumoured to be what Mrs Wallace used on a less than well endowed Prince Edward and why he would not leave her and subsequently abdicated).
For guys you can rub a woman's g spot just by flexing your PC muscle.
As a fascinated teenager I started by lifting my face cloth with my erection . building up to a bath towel then a heavier wet one.
So yes I really advocate it.