What Is Amazon KDP Select, and Should I Enroll My Book?

I’m preparing to publish my book on Amazon and came across KDP Select, but I’m not sure if enrolling is the right move. From what I understand, it gives authors access to Kindle Unlimited (KU), promotional deals, and higher royalties in some regions. But the catch is that I’d have to make my eBook exclusive to Amazon for at least 90 days.

Has anyone here used KDP Select before? Did it help with sales and visibility, or did the exclusivity limit your reach? I’ve also seen some book marketing services in USA that assist with KDP promotions—do they make a big difference?

I’d love to hear from authors who have tried both KDP Select and wide distribution (selling on multiple platforms like Apple Books, Kobo, and Barnes & Noble). Which approach worked best for you?