Y i need wifi to back up my phone??

Wifi is how the data is transferred from your phone to the cloud where your backup data is stored, if you have enough iCloud space left. This is also how cloud data is used on your phone.

Leaiphone13 wrote:

y need the wifi to back up my iphone data

Because wifi is a requirement to back up to the cloud, unless you have an iphone 13 then you can use data.