The Raspberry Pi 4 is an extremely versatile microcomputer with a wealth of applications. It can be used as a Media Center, as a video surveillance system, as the heart of a custom project or even as a small personal computer.
But by itself the Raspberry Pi is “naked”, it’s a single circuit board with a number of connectors that requires an external power supply, a microSD card, and some other peripherals to make it useful.
If you’re just experimenting with the Pi or using it as part of a larger project then a case may not seem that important. But if you’re going to be using it as a stand-alone device for a permanent application then you’ll likely want to find a nice enclosure for it.