Some of them already have an established job and will not try to pursue a professional career. The rest of them are facing a dilema. These players have to play and practice full time to hopefully be recognized by a team. This isn't really compatible with a regular job.
This is where boosting and coaching logically find their place. Boosting makes their skills useful and it allows them to keep on working towards their goals. Most of the pro and well known players have been boosting on their way to who they are today. If we take the exemple of World of Warcraft, up to 95% of the pro players have boosted before. This is a very common step and it's helpful to the game editors and their e-sport scene because it brings a chance for every aspiring competitive player to work while playing the game they love.
Furthermore, people getting boosted are mainly located in the USA and western Europe. Whereas players who provide boosts are from everywhere, including eastern Europe. For them, boosting means much more than the average salary in their country.
So even if you don't understand why would a person buy a boost in a video game, it actually helps other people make a living in an unstable industry.