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Apple allows you to restore to the latest iOS versions only to prevent users from downgrading to an older iOS version, which would have made it easier to jailbreak the iOS device. For example: If Apple allowed users to downgrade their device, they could downgrade back to iOS 8.1.2, which was the last iOS software update that could be jailbroken using the TaiG and PPJailbreak.

Apple stops signing the firmware files (by not providing SHSH blobs) for older iOS versions to prevent users from downgrading. Tools like TinyUmbrella have allowed you to save the SHSH blobs, so you can use it to create (stitch) a pre-signed IPSW file, which could then be used to restore your device with an iOS version that Apple was no longer signing.

Apple has closed the loophole that allowed users to use the SHSH blobs in newer devices (iPhone 4S and later) and also in iOS 7 or later. So you cannot restore to 6.x using SHSH blobs except on iPhone 4, iPhone 3GS, and iPod touch 4G (and iPad 2 to iOS 5.x if you have iOS 4.x and iOS 5.x SHS blobs).

Despite the lack of support for new iOS versions, it is considered a good practice to take a backup of the SHSH blobs, so if someone discovers a way to hack the restore process in the future to again using the custom IPSW firmware file, then the SHSH blobs would come in handy.

Semaphone, developer of TinyUmbrella has recently released a new version of the tool that supports iOS 8.2 or lower. He has indicated that it maybe possible to downgrade to older iOS version using the SHSH blobs.