Why would I want to turn on iTunes Match on an iOS device? I'm a bit confused about iTunes in the Cloud vs. iTunes Match. 
My understanding is that iTunes in the Cloud allows me to have my songs in the cloud. I can see all my songs by turning on "Show all music" on my iOS device.
iTunes Match allows me to add music to the Apple cloud that was not bought in iTunes. I have enabled that on my Mac to add music I buy from Amazon.

But why is there an iTunes Match switch on the iOS device? I cannot buy music on iOS and add it to iTunes and turning on "Show all music" already gives me all songs.
I agree that they make it confusing in their advertising. I think part of the problem is that they have blurred the lines quite a bit recently, leaving one key difference: music purchased with iTunes vs. that purchased elsewhere. iTunes Match uploads your music and puts it on all of your devices regardless of where you purchased it, while iTunes on iCloud just does that with music you purchased from iTunes.

For example, I have a number of songs that I purchased from Amazon, and they consequently don't sync to my iPhone even though they are on my computer. This would apply to music ripped from CDs as well.