"What are you eating?"
You are right, if it is a set phrase, there is no need for reason. If it is a phrase in Slovakia and Czech Republic then it might be a regional thing somewhere near the borders.
Unfortunately I could not get any google or "NKJP" result for this phrase, all came back was either "po co"- why, or misspelled "jest", or "jeż"
I think more common, at least in my area, (not that far from Slovakia), is "Co tam jesz" - literally what do you eat there, but "tam" makes as much sence as "to".
I thought the difference was similar to English "co/what" (fully open question) vs "jaki/which" (limited number of choices) ...
Obviously what/which can be used almost interchangeably in many situations. But to me they each have a more 'prominent' meaning, as above (may be an idiosyncratic distinction).
Maybe it was naive to think it was that simple
"który" means "which".
"jaki" means "what", "what kind of". As in "What bread do you like?"
"co" is the simple "what", as in "What are you doing?"
See me quoting br0d4 here: https://forum.duolingo.com/comment/12283728/What-is-your-answer