Playtomic ratings are primarily there to get your money via gamification. It pushes you to increase your rating by playing more matches, and Playtomic makes more money through that. The whole "get better, compete, and find matches according to your level" is the marketing cover of a sneaky algorithm.
It also gets more difficult to increase your rating as your reliability increases so you spend more money to increase faster. You can do leveling with a coach, which bumps your rating up, but sets your reliability to 50%, quickly sending you back to where you started if that is not your actual level (for most people, it is not). Then the whole grinding on the hamster wheel starts again.
On a professional level, Playtomic rating means nothing. Some countries do not even use Playtomic. They just rely on how people judge themselves and book matches through Whatsapp, Viber, etc.
On a recreational level, it is very hard to standardise the system.
Maybe what could help for more transparency and easier matchmaking could be Playtomic ranking players based on number of losses and wins per country/city/club. Then if I want to book a certain match in a club, I would know how I compare against my opponent. Introducing statistics like Winners, UE, and FE would also make it more precise.