What's the use of icloud

The iCloud is for backups, storing your data and files in the cloud, and for syncing your data and files with your iPhone, iPad, and Mac etc. and every other Apple device. If you use your iCloud account, when you buy your next iPhone, you don’t need to send your photos etc. from the old iPhone by manually but rather it will sync automatically from the old iPhone onto your new iPhone.

for more information: Introduction to iCloud - Apple Support

iCloud is used to store photo's files and messages to save and clear up storage. You don't need an iCloud and whether you set it up is purely a choice of preference.