Whats the point of buying an album on itunes when you have Apple Music?

Should you ever decide to stop paying for Apple Music, buying the album insures you can continue to play it after the Apple Music subscription has expired or has been cancelled.

Also this has nothing to do with iTunes Match (the forum you posted in), so I'll request the forum hosts move it to a more suitable place.

In addition to the above, it can be used in more places, including iPods other than the iPod touch and iOS devices incapable of running iOS 8.4 or above.

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Assuming you intend to keep AppleMusic for the long term, very little.