Plenty of users navigate the web with nothing but a keyboard — tremor, a temporary injury, a switch device, or simply no mouse at hand. A custom component built to only react to hover or click (a hand-rolled dropdown, a div wearing a button's clothes) is a solid wall for them.
The baseline is simple: Tab/Shift+Tab moves between elements, Enter or Space activates one, and arrow keys move within a list or menu. Real native elements — `<button>`, `<a>`, `<input>` — already ship this behaviour built into the browser, so there's nothing to reimplement.
The demo moves through a 4-item toolbar with Tab, arrow keys, and Enter, with key badges popping up alongside each move.
When to use
Every time you build a new component, try using it start to finish with nothing but Tab. Wherever you get stuck is exactly what needs fixing.