Citrea's case is rather unique: it is a technical requirement for them that a specific transaction in their presigned transaction graph contain 144 bytes of "arbitrary data" (data relevant to the light client protocol used by Citrea's bridge) -- this data MUST be put onchain in a single tx, which means that the envelope technique used by inscriptions will not work because envelopes require two transaction (a commit and a reveal). To work around the 80 byte OP_RETURN limit Citrea is using one 80 byte OP_RETURN output and two "unspendable" Taproot addresses that each have 32 bytes of data embedded directly in the address, combined creating the 144 bytes needed. Citrea devs have publicly said they would use a single 144 byte OP_RETURN if they could, but are going with the workaround in the meantime.

To the broader question about "when would someone use OP_RETURN instead of witness space e.g. envelopes", Vojtěch Strnad did an analysis and his conclusion was: