Immutable distros are great for applications where you want uniformity for users and protections against users who are a little too curious for their own good.
SteamOS is a perfect use case. You don't want users easily running scripts on their Steam Decks to install god knows what and potentially wreck their systems, then come to Valve looking for a fix.
Immutable distros solve that issue. Patches and updates for the OS roll out onto effectively identical systems, and if something does break, the update will fail instead of the system. So users will still have a fully functional Steam Deck.
If you're not very technical, or you aren't a power user and packaged apps like Flatpaks are available for all your software, then go for it. I prefer to tinker under the hood with my computers, but I also understand and except the risk that creates.
Immutable distros are a valuable part of a larger, vibrant Linux ecosystem IMO.