Why cheat? Why indeed?
There's probably as many reasons as there are people playing Oblivion, and each has their own reasons for doing it. Also, what may be considered a chaet by one, might not be by many others. TCL, for example, will allow your character to pass through walls and such to be able to retreive items that have fallen through glitches in the game to "the nether land" (as I call it). It's also a great tool for when you want to take that nifty screenshot of your character on the top of a tree, or on top of one of those unclimbable Ayleid arches. Or it will allow you to float just out of reach of that nasty monster while you throw flameballs at it
After you've gone through that beginning dungeon, half a dozen times, do you really want to risk your character dying?? Expecially after you've just spent nearly 1/2 an hour fiddling with the facegen to get his/her face just so? I don't. And with my type of dungeon delver mindset, I also want to drag as much loot out of the place as I can. No matter what the backstory one has on their character, they are still an escaped prisoner with nothing but some rags to wear. They're gonna need to gets funds somehow. TGM makes this a whole lot easier. encumbrance? Wounds? Lack of Majicka? Pah! Those are for mere mortals! ;P (seriously, you should see the pile you can make at the end of the starter dungeon - just make sure you bring all of it outside before 3 days time are up). So, yes, I do cheat through the starter dungeon for reasons of speed, saving my work, giving my char a bit of a financial hand, and the been-there-done-that factor. TGM is also nice if one is playing a race that is technically immortal such as a Dremora or Xivilai.
After the starter dungeon, reasons for cheating depend on why I've gone into the game to begin with. If it's to seriously play, personally I try not to cheat. I'll use TGM to get loot out of cleared dungeons, but once again, that's more of a time saver for me. I don't consider bags of holding, summonable chests, and summonable doorways to toehr realms with storage containers reallys cheats as Tamriel is a land with magic and if your character has the magika well,...
And if one has companions, one can get them to carry stuff as well. If I'm playing for the sake of just messing around and screenshots, I might cheat like there's no tomorrow. and as I don't save on top of my old games (makes for a ton of savegames) I can always go back and play "for real".
Using codes to get through quests. Now that is something I wish I'd never have to do. It is the ultimate and worst type of cheating in my opinion. But, it is at times necessary even with the patches (official and unofficial) as many quests still will not start or finish properly. And, if you were unlucky enough to get a bad disk, you may have a lot of these.
To the Void we all shall go...