Who should hire a coach for an endurance sport?
Anyone who wants to perform well.
Should I buy that new bike or hire a coach?
Both! (lol just kidding) do whichever will make you happier, but hiring a good coach will make you faster.
Why would I hire a coach when my fast friend shows me the workouts he/she does?
Coaching is all about the timing, when to apply training load, when to rest, and when to push different physiological systems. Unless your fast friend is also prescribing you a schedule, they probably aren’t helping you go faster on race day.