Monthly charges $9.99

It is a monthly charge. You are renting the storage. You have access to it as long as you pay for it.

It clearly states its a monthly charge:

Here's how it works

from https://support.apple.com/en-us/HT201318

Choose from a total of 50GB, 200GB, or 2TB. You can even share the 200GB and 2TB plans with your family. Your storage is upgraded immediately, and your monthly payment date reflects the purchase date for your plan. If you upgrade from one paid plan to another, we’ll cancel your existing plan and charge you the prorated cost* of your new, larger plan.

You're billed every month for your storage plan unless you downgrade or cancel your plan. Each month you'll get an email to remind you of your upcoming payment.

Because iCloud is not free. If you increased your iCloud storage to one of the higher options like 50GB or 200GB or 2TB it will cost money.

You can cancel it if you don't want it, but make sure you download anything you don't want to lose from iCloud as after cancellation anything you have that exceeds 5GB will be deleted.

See here: link-> Downgrade or cancel your iCloud storage plan - Apple Support