Collared t-shirt is like a mix breed of shirts and t-shirts. T-shirts are far too casual for men after a certain age (read, college days!). After that it is preferable to wear a jacket over your t-shirt to avoid looking immature. However, that is not really fair for men and thus we have the collared t-shirts.
1) V-necks and u-necks suit some people and not so much for some others. Collared T-shirts are universal. They suit every body type.
3) It gives your body a structure as the fitting of collared t-shirts is very cool. Makes you look slim and fit.