Focus Interrupt Sounds (FIS) plays a sound when your target is making an interruptable cast, and you have an interrupt available.
It DOES NOT cast the interrupt for you. I agree that would be botting.
If you install it, please, PLEASE, disable the party chat option to announce the interrupt. Anywhere except in a group of consenting adults in private, that is hella annoying.
Other addons are to taste.
Personally, I regard placing my frame and my target’s frame somewhere I can see them without dislocating my eye-sockets as a deal-breaker.
I like seeing a separate cast bar. I like seeing timers on my CDs.
I find Auctionator a must for the AH. Even Ion suggests it. I regard the more advanced TSM as tantamount to botting at this stage of sophistication.
I hate the new nameplates since Legion. Replacing those is a must, for me.
TomTom or Carbonite are required if you want to find things you look up in comments on Wowhead.
Postal means never accidentally sending something valuable to a complete stranger.
And so on.
Some addons are just minor conveniences. I find SellGrey (which can actually be done with a macro) and QuickRepair, for example, very handy.