The more XP, the higher the level.
The next thought is more important: What are levels for?
In certain levels, equipment is released for buying. Usually equipment of higher level is better equipment. Thus, we have the opportunity to buy equipment of a certain quality at a certain level, the alternative being to wait for a higher level to buy better equipment.
For price & cost concious users it is important to know when a possible buy is necessary and helpful and when it is a waste.
Welcome in capitalism!