First off, you can pay robux for coins and gems, both of which capable of purchasing some broken shit
Secondly, there's a wave restart mechanic that requires tickets that you pay for with robux
Third, there's a wheel that gives rewards but can only be spun if you pay for a ticket with robux, and the only free way to get said tickets is on a select few days on the daily login rewards
Fourth, the highest tier ability crate requires robux to purchase
Need I say more?