Disclaimer: I do find the other answer from Philipp to be excellent
I approach this from a much different point of view. I'm going to focus on Magic The Gathering: Online (Mtgo)
Mtgo is something with many (illegal) free competitors. First, it is illegal not because of the client/server, but the assets that are under copyright and the game rules itself (I believe.) There are people who play these, but there's a big second point here:
If you're hosting tournaments for this game and other companies can't because (while your source is free) the assets are copywritten, then you have the only legitimate tournament platform. Tournaments in Mtgo are very well designed and influence the buying of product in game by giving hints of the workings of the metagame. These results (or at least, some subset of them) offer prestige on their website by showing you're good at the game with your build, or that you've come up with a new deck that eats the current meta-game (gaining prestige for being the first to find a new approach.)
While you're probably not doing a trading card game; it could be tournaments allow you to gain good items or people in the Top-X% of players within the last Y-days have special blessings (not even items!) that give them bonuses, abilities, etc.. making content easier, or accessible, etc.
Anyone can print sheet after sheet of fake magic cards and attempt to play, and they do! But they also buy the real product, want to play in the real tournaments (that require the real product), and want the prestige in that community.
NOTE: And Mtgo is famously bad as a platform. Despite being a glitchy, laggy, terrible user interface, garbage fire of a piece of software; it has entire businesses built around it for the secondary market, continuous tournaments all-day every-day (that are utilized by hundreds of players), professional grinders constantly tweaking their builds via the tournaments and twitch streamers and blogs and My Oh My is that a lot of community, secondary businesses, etc for something that is a true masterpiece of mediocrity.