Field names have to be hard-coded into a formula, which makes it so you can't do something like indicate which field name to pull from in a multiselect field. You can only use a switch with hardcoded field names, or use a whole bunch of if statements to make different things happen depending on the multiselect options. I'd love to be able to write like:

FIELDS({multi-select that says which fields to return})

and have that return the value of all those fields as an array