Why need iTunes Store in concept?

Purchasing albums and purchasing tracks means you’re somewhat less prone to losing access to your preferred music, where a music rental service (such as Apple Music) can and sometimes does change what is on offer and that for various reasons. Major artists and labels probably won’t shift in availability. Smaller labels and lesser-known artists can sometimes become unavailable.

You might also prefer a particular version of an album or track, and sometimes an artist or label will remove that version and will replace it with a newer or different or remastered version. If you don’t mind that change happening, a rental service can meet your needs. If you do, you might want to purchase and archive your preferred copies.

Music purchased from Apple typically also has no DRM, which means you are not restricted to playback with devices compatible with Apple Music; you can transfer and play back your purchases on most audio devices.

The Apple Music subscription is a rental, and not a purchase.

Stored offline or not, the music rented from and downloaded from Apple Music subscription will be inaccessible, if (when) the Apple Music subscription expires.

This due to the use of time-limited DRM; of encryption.

Apple makes this all somewhat more confusing with their re-use of terminology, too.

Apple uses “Apple Music” as the name of the app—the Apple Music app which can play music ripped from your CDs (and a Mac with a SuperDrive or other optical drive can rip from those CDs), can play music purchased from the iTunes Store and from elsewhere, and can play music rented from Apple.

Apple also uses “Apple Music” as the name of the Apple music rental and streaming service Apple Music the subscription.

And for extra added confusion if you’re using Mac, the iTunes Store is available inside the Apple Music app on macOS.

But again, Apple Music the subscription is a rental, and not a purchase. Stop renting, and the music (and the iCloud hosting of your own music that is part of that service) all disappears.

The iTunes Store (and other music download stores such as Amazon, Bandcamp, Beatport, etc.) sell music downloads that you can add to your "permanent" music collection.

If you have a streaming subscription to Apple Music, you may very well not feel a need to purchase any tracks. Some users do, some don't.

The Apple Music app is limited on iPhone and iPad.

Particularly around ripping and importing, among other limits. (macOS and Windows can rip and can import just fine.)

Though not as limited as iPod was, it’s less than fully capable for entirely standalone and self-contained operations.

Apple intends to have your music library stored elsewhere; either hosted in iCloud (via the iTunes Match sync service or the Apple Music subscription service and its included sync service), or on a Mac or Windows system.

This all as differentiated from subscriptions / rentals, and purchases / rips.