It is, and Monero tx fees are pretty low right now - but as it continues to gain adoption, fees will rise. Cashu enables microtransactions and zaps for small amounts. Its not essential, but it can be fun, and a good way for newcomers to get a taste of freedom at trivial cost. Yeah, its custodial (but trivially anonymised), but that just means a different threat model - its small change in a pocket, not a wallet.

That is the only explanation for Cashu that makes sense to me. But I worry that people will get comfortable with it and forget this, especially if it's mass adoption we are talking about. Although an easy fix would be to hard code a limit for your total balance.