I've watched multiple streamers play this game and literally every single one of them did this, you are not alone Dan

This is like asking if you're going to play whatever the next Fromsoft game is

Basically there's no intention for the average player to use it. It's just part of their process of slowly transitioning the game away from heavy addon use, which is something they've actually been working with major addon creators on to choose what does or doesn't get integrated into the base game

Since Blizzard is phasing out the ability for addons to have access to this information, they kind of have to provide an option within their own system to do the same thing. The GCD penalty means you'll do better DPS if you actually know your class but the option is there if you need it.

It's for extreme casuals and people with disabilities. They're trying to reduce the playerbase's reliance on addons. Helkili is an addon that already exists which does the exact same 'highlight the next button you should press in your rotation' and is popular for accessibility reasons.