What's iTunes?
Media player from Apple
With iTunes, you'll have an organized place with all your music, movies, series, podcasts, and audio books. You can play all these files in iTunes. That's useful, because you'll have 1 program to manage and play all your media files. Apple iTunes doesn't work together with all streaming services, so you can only see the files that you've added or bought in the iTunes Store. From macOS Catalina onwards, you can't find iTunes on the Mac anymore. You can listen to music via Apple Music, for example.
Synchronization of your Apple devices
If you don't use the iCloud, you'll need iTunes to synchronize your iPhone or iPad with your Mac, desktop, or laptop. You can also use iTunes to make backups. These backups are stored on your Mac, desktop, or laptop. Is there something wrong or did you buy a new Apple device? You can easily restore everything. You can do that by connecting your iPhone or iPad to your Mac, desktop, or laptop with a USB cable.
Note: from macOS Cataline onwards, you won't find iTunes on the Mac anymore. You can manage and synchronize your Apple devices via Finder.
Storing music, movies, and more
In iTunes, there are various tabs where you can place and find your files. There's Music, Movies, TV Shows, Podcasts, and Audio Books. You can also add your own files. If you purchase things in the iTunes Store, it's added to iTunes automatically. Do you buy a music album in the iTunes Store, for example? It's added to the Music tab automatically. In addition, you can find the album on your iPhone or iPad in the Music app.