Lightroom gets a "minor" revision around once every three months, to keep up with new cameras and to "fix" bugs. It's free for owners of the current version.
As to why some retailers offer it for a discount, well that's up to them -- they just want to keep their sales up. It will be a while before LR5 comes out so I wouldn't sit around, instead try the free trial, good for 30 days, just make sure you do enough "homework" about the LR organization features.
I did not know about the revisions. I take it you go to adobe site and download the revision or update and if I buy one will it be ready to go for a 5d iii or do I have to get the revision after I install lightroom on my computer ?