Here's relevant post. IMO their motivation is either data collection or disrupt Bitcoin development. Claiming "free" token/coin usually require your email address and accessing their website (which means they obtain your browser fingerprint). On extreme case, they might ask for KYC verification.
Data collection would mean they get really a lot of money for a few stats about their site's visitors. Again, not sure a few ads and browser information is really worth more than mining and giving away worthless test coins. Disrupting BTC development makes a bit more sense to me since it's not monetary, like BlackHat said. I have had a small web search for 'buying testnet coins' and it yielded nothing, so I don't think that's how they make their money.
not sure if it has some limitation that would incentivize using testnet instead
The only serious limitation in signet I can think of is mining.
Yes, it's true, but on the other hand - let's say I'm developing a miner and I want to test it. The difficulty on testnet is probably way too high to get quick and consistent results to see if my driver and software is working fine, anyway. Then I would probably just setup regtest and mine in there. (not sure if this is possible but I think you can mine on regtest)