Buying iCloud space

You pay every month because that's how Apple designed it. You pay only as long as you need the added space. It starts at 99¢ a month for 50 GB. Any time you no longer need 50 GB you can stop paying and it will revert to 5 GB.

See this Apple link for details-->Buy more iCloud storage - Apple Support

Note that buying iCloud storage will not increase the storage capacity on your phone. Your phone is stuck with whatever it had when you bought it.

Apple no longer offers a yearly plan so you have to pay every month. I’m assuming, even though I’m not supposed to speculate on Apple policy, that they went to a monthly plan to make it easier for users to upgrade or downgrade as needed and without having to worry about being tied into a yearly plan and request refunds if they downgrade.