Comment by Tcepsa

It's for letters/numbers. The mechanical keyboard community has adopted phrases like "chord" and "arpeggio" because they refer to analogous things in the typing world ("pressing multiple keys at the same time" and "pressing multiple keys in quick succession", respectively).

In keyboards with a limited number of keys (such as in TFA) they become especially crucial to being able to express the full complement of "standard" letters, numbers, and symbols.