So there's additional data that can be included in transactions OPRETURN field that can be discarded once the tx is hashed so databases won't be bloated.
The only downside I understand is the potential for including exogenous value that would tempt double spending / a chain reorg.
I know next to nothing (wasn't even aware of Citrea and their zkrollup dreams) but I think OPCAT would be quite nice to include for sidechains and all that jazz. I imagine Citrea would be able to use that for their purpose too.