With my present recorders I can tune stations with numbers 1-100 if they are not paid extras, I normaly use a Cable Box; what if anything do I gain by getting a recorder with a digital tuner?

The new recorders are required to have digital tuners now, they have to have at least OTA but most include QAM cable as well. It means you can record any digital channel in SD resolution onto a blank if you want to, and you don't need to pay extra for a cable box rental if you don't care about true HD resolution as all the tuners in these things donconvert to 480i max. The industry is really afraid of anybody being able to make full res copies I'd suspect so for now thats what we get.
You can always get a hd cable box with a hard drive but most of those wont let the hd signal out to any type of recorder for saving to disk for later playback so it depends on what is more important to you I guess.
I'm going to get one as I can at least record anything that comes in in the clear, plus soon the analog stuff will be gone and you'll need some kind of digital tuner in something.

The new ATSC tuners will mostly benefit those
of us that get our TV OTA.
If you are getting satellite or cable you don't
really need the digital tuner.
Since the March 1st deadline, some recorders
are being sold with no tuner at all, rather than
conform to the new ruling.

If you dont want to rent a cable box then you can use the digital tuner in a machine that supports QAM to get all unscrambled channels, that's about it.
Of course you can also record those channels but if you have a PVR type cable box and dont care about making disk copies later then yes, it would mostly be for those using OTA to get the local digital channels.