I always turn on iCloud on clients devices ( computers , iPhones , iPads ) for the following reasons:
iCloud will keep a copy of your contacts , calendar information , reminders , emails , photos and documents ( created with pages , numbers and keynote ) in the cloud
Its All About The Sync
Get an iPhone and only ever had an iPad , iCloud will get and sync your information with your new device.
Change a contact on one device , its changed on all devices
Change an appointment on one device it’s changed on all devices
Need to get into your iCloud account from another computer , just log into www.icloud.com with your Apple ID and password and you can access all your data.
Where Are My Devices?
Want to know where your devices are ? Just log into iCloud and if you have turned on ” Find My iPhone / iPad: you can see the location of your devices.
Really handy if they have been stolen or misplaced.