This is a difficult question, I think it depends on the person as to what they would want to increase. In some cases it could be that a man having to use sports showers or something would be embarrassed by letting his team mates seeing him in the nude if he is unusually small. Most men worry about this anyway and sometimes take a sneaky look to see how they compare. Most realise they are worrying over nothing but some men, even those who have large erect penises do have a small flaccid penis.

With these extenders you do as a rule gain both flaccid and erect size but there is a way to just grow the flaccid penis if you really don't want to gain any erect size. This is done by putting the extender to a smaller setting, still stretching the penis but nowhere near the limit. This will eventually train the penis to stay in this slightly extended state. This will also gain faster girth for the flaccid penis as it's not stretched out so far.

There doesn't seem to be any way to just gain erect length but I can't think of any reason why someone would want that anyway. I think what it comes down to is are you worried about people seeing you in the nude or worried about your abilities to please your partner? Either way these extenders can help, especially if you're willing to put some work into it.