As a woman who spent the greater part of her life as a small-ish B cup but is now (due to weight gain) a large-ish C cup, I have to say that I love having larger breasts. I just like the way they look and feel. The only time that the observations of other people come into it is when I am actively trying to engage other people in looking at them, lol. I can't speak for anyone else, of course, but competition has nothing to do with it.
As for men not liking implants in real life, I have to differ. There are plenty of guys who dig perfectly round, perky implants. Just as there are plenty of men who prefer 'itty bitties,' 'hangers,' and 'milk jugs.'
It's just a matter of an individual's taste. As for why women get them, I think the reasons vary between self-image needs and wanting to look more 'desirable.'