IMO (International Mathematical Olympiad) are extremely complicated problems that even professional mathematicians often struggle with them. IMO is a level on its own - Gold Standard. Not all Mathematical Olympiads are of the same level of complexity as IMO. Good example are Olympiad caliber problems that are not overkill - Hungarian Problem Books.
MO problems are useful, because they make you think out of box, they often times involve several branches of math in one problem, such as number theory problems go in hand with combinatorics; they don't require complex mathematical machinery, and technique of solving problems, directly translates to solving complex problems in analysis/abstract algebra.