why need wifi for backup, cant u use mobile data?

Hi izaman8591

Welcome to the Apple Community.

Re: why we need to be connected to Wi-fi for the backup to iCloud

Seems iPhone models that support 5G (see link below) CAN use cellular data to back-up to iCloud IF the provider supports 5G

Models that support 5G – Apple Support

How to: Use 5G with your iPhone – Apple Support

Discussion thread on similar topic:

Can I use mobile data to back up my iCloud - Apple Community

says:

"Read Apple's note regarding iOS backups via 5G::

On models that support 5G your carrier may give you the option to back up iPhone using your cellular network.

Go to Settings > [your name] > iCloud > iCloud Backup, then turn on or off Backup Over Cellular."

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Re: does the backup starting from 0, which mean phone will backup my data from scratch

Sorry I'm no expert ... and as we are all Apple users, like yourself, in this community we cannot speak for Apple

Seems Apple says that older back-ups can be deleted if you want,

Yet only the data actually on your iPhone (nothing you have deleted from your phone) remains backed up.

Here's a discussion thread with more details that may help clarify:

If you've backed up to icloud once before… - Apple Community

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More info. if needed: What does iCloud back up? - Apple Support

How to: Manage your iCloud storage – Apple Support

What's the difference between device storage and iCloud storage? - Apple Support

iCloud Support Webpage:

iCloud - Official Apple Support

All the best :-)