How does iCloud work?

iCloud is a storage space on an Apple server somewhere. You get 50GB for 0.99cents a month.

You can sync data such as Photos, contacts, calendar, notes and email to the iCloud storage, and you can store iOS device backups there as well.

Weather its worth it, only you can determine based on your off device storage needs.

iCloud will not affect your device's local storage capacity in any way.

Hello there,

iCloud+ plans and pricing - Apple Support

This is all the details about iCloud plans. I have the $0.99 plan, and I think it's absolutely worth it, but it depends on how many devices you have, how many photos you take, and how much you want to share between your Apple devices.

Hope that helps!