What Brad said. For me, the most compelling element is having your entire catalog in the Cloud, and being able to access it across any OS, tvOS, iOS device - for streaming or downloading.
To boot, is the "Match" feature, which is nice if you have files in lesser quality than what is available in the store, such that upon your tracks being "Matched" (iTunes scans your files, and based on digital footprint determines whether or not the track is a known track that is available in the iTunes Store) you then have access to iTunes Plus 256Kbps AAC versions of the files.
A final note: In the instance you want to "Upload" a track to the Cloud but don't want it to be "Matched" there is a workaround. It's a bit of a pain in the butt, but I'm happy to walk anyone through it who would like to try it.
The reasons for not wanting to Match can be many, including say, wanting to upload an MFSL version of an album, and iTunes ends up "matching" some or all of your tracks to the version available in iTunes (not MFSL). In these cases, by way of a bit of trickery you fake iTunes into not being able to recognize the tracks, and in doing so iTunes does not identify the track in question, and instead of "matching" it, it "Uploads" it, and your uploaded version of the track is then available in the cloud. Another example is the case of compilation albums... If you rip a greatest hits cd, and it's been mastered differently than any of the given tracks on the album compared to their original album release versions, you never know what iTunes will end up matching each track to. In some cases you end up with DR all over the map, or end up with different masterings, etc.
An additional application for wanting to "Upload" is rooted in my anal-retentive, wannabe audiophile tendencies. While any "Matched" track is matched to iTunes Plus 256Kbps AAC quality, iTunes will actually "Upload" (any non-matched tracks) at a bitrate of up to 320Kbps, whether MP3, AAC, VBR AAC, etc. So, in the case of me wanting the highest quality versions of my music available in the cloud, I "force Upload" tracks in 320Kbps (VBR) AAC [iTunes encoded], and end up with a higher quality encode than the "Matched" version available in the store.
Truth be told, I have my entire U2 catalog force uploaded in 320Kbps (VBR) AAC, with the exception of tracks that are not available losslessly, and/or may only be available in iTunes (iTunes exclusive content), or in the case of the recent "Mastered for iTunes" releases, I'm going with those for listening as opposed to my previously uploaded 320Kbps (VBR) AAC versions ripped from cd.