Because they’re fun and also because players get to win Medals and Token Rewards.
Keep in mind that players can only play Tournaments with an internet connection.
Also, don't forget that Tournament Medals and Prizes are awarded only when the Tournament in question is finished, according to your rank at the moment when the Tournament concludes.
Tournaments run on stamina, but it could run out!
Out of stamina? No worries! You can either refill it with Tokens or wait and it will automatically get filled with a bit of patience and time.
Even the greatest PAC-MANiac has to take a break, right?