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.

Chords and arpeggios apply to typing as well, is how stenographers type so fast.

There's all kinds of text keyboard mappings including some like court reporter's input devices.

i don't actually know a lot about split/ergo/mech's and don't know of any non-rabbit-holey info sites but you can learn a bit about 1 and 2 handed split ergo's and ZMK/QMK firmware mappings at https://old.reddit.com/r/ErgoMechKeyboards/

ZMK's docs are pretty good https://zmk.dev/docs/keymaps

It's for text input, I believe, but their use of the icon (in the title) certainly adds confusion.

I'm also wondering whether the presence of arpeggios and rolled chords is a benefit, or if it makes it harder to pick up. Eg tentatively assembling a chord one key at a time because you're learning must look like a rolled chord, right?