I am sure that many of us have seen guys at weddings or other formal functions, wearing an ill-fitting tuxedo that they have no doubt rented for the occasion. When you think about it, it actually takes away the whole point of the formality. Such situations occur when the wearer is not a usual function attendee, and has maybe been forced to wear what he is actually uncomfortable in, and certainly would not normally buy.
But, actually, it does make sense to purchase a tux rather than rent and look a bit silly!