why to purchase more icloud storage space??

If all of those devices are using your AppleID, then each of their backups and other files are going into one iCloud account storage space. iCloud is used for the data you sync from each device (contacts, notes, calendars, photos) and for each devices individual backup. If all of that exceeds 5GB (the free iCloud account limit for any one AppleID) then you need more than 5GB for it all.

nesa961 Said:

"why to purchase more icloud storage space??: Why do I have to purchase more iCloud storage space if each of my apple devices have a sufficient storage space?"

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Upgrading Not Mandatory:

iCloud Storage is a service offered by Apple. No one says you "Have to purchase" it.