Storage is not just a container. Both computers and phones use spare space as working room: for temporary files, for updates, and as an overflow area when memory runs short. When free space disappears, that working room disappears with it, and the whole device slows down.
Much of what fills a drive is invisible in normal use. App caches, old update packages, message attachments and offline copies of cloud files accumulate quietly. A photo library that lives 'in the cloud' may still be keeping full-size copies locally.
Deleting the wrong things is a real risk. System folders, program files and anything you did not create yourself should be left alone. Almost all safe space comes from your own downloads, media, and the cleanup tools the operating system provides.
A useful rule of thumb is to keep roughly a tenth of the drive free. Below that, updates start failing and performance falls off noticeably.