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ICloud photos will upload and store any photos you take on your iPhone into the iCloud, meaning you could potentially view them on your PC or Mac.
If you do not have any other Apple devices and do not have a Windows PC with iTunes/ICloud installed, then turning off iCloud will do nothing.
However, being that you might want to swap to another iPhone at some point in the future, and being that Apple gives you some iCloud space free of charge, you might as well leave it on to ease a transition between iPhones when you upgrade. Also, if you lose your phone, or it gets damaged beyond repair, keeping iCloud switched on means you haven't lost your precious memories.
Here's a link to an article on iMore that gives you the pros and cons of iCloud.
https://www.imore.com/should-you-use-icloud-photo-library
Stay a while, Sign up using the link Annie posted above, and you'll find out more incredibly useful things about how to really get the best from your phone.