I don't know if I need this service

Hi - Bear in mind this is not additional phone storage - this is iCloud storage. They are not the same.

See:

https://support.apple.com/en-gb/HT206504

OK, which service?

Hello,

It depends on how much data you stored on your iPhone. Before worst case happens to your device always make sure to have a backup of your device on PC or iCloud.

To determine how much storage you need for your device follow this;

Settings > General > iPhone Storage and determine how much storage is in use for your personal files, datas, photos or apps.

You can learn more via;

How to back up your iPhone, iPad, and iPod touch

https://support.apple.com/en-us/HT203977