What do you want in your stack?
Because Linux is modular, a lot of functionality is duplicated by various components. For example, modern filesystems try and be all things to all men, while other components such as Device Manager and md-raid are more single-function components.
|Which component provides what|
|Component||Combine Devices||Redundancy||Integrity Check||Snapshots|
Why not btrfs (or another modern filesystem)
Looking at the table, and seeing that btrfs provides all three capabilities, you might wonder why you want to bother with raid and lvm - why not just use btrfs, or another modern filesystem. The reality is that not all filesystems provide all the features you may want, nor will they necessarily provide them in the way you want.
Take btrfs for example. You may want to have several mount points, so that filling up say /home doesn't cause your mail system on /var to fall over. Btrfs provides redundancy - but only raid-1 is reliable at present. That said, btrfs applies redundancy at a far finer-grained level than can be done by lvm or md-raid lying underneath the file system. The current strengths of btrfs are perceived to be snapshotting and integrity checking.