What lots of people like about ResHacker is that it lets you see the .lnz as it's supposed to be, laid out like the text file that it really is, and then you can edit it to any size you wish. ResHacker re-compiles the file, you see, whereas it's a horrendously long and laborious job to re-jig everything to a different size in a hex editor (so we don't even try -- we make the .lnz fit the space).

There are some down-sides to ResHacker; the main one is that it can cause corruptions, and another big one for me is that you really are not fully in control. ResHacker does so much to the file that it's a nightmare trying to sort out problems with a ResHacked breed -- and I assure you that I know this, I've been sent plenty of the dratted things by tearful breed-makers. 

Here comes the "plug" -- in my view, my "easy-edit" packages are much better for editing. If you're making litterz, you need no tool other than Notepad (not even a hex editor), and you can change the breed while the game is actually running; it's as close to instant viewing as you can get! You can edit to your heart's content in Notepad, and you can easily see what's wrong with your hexie. When you are finished, and if you want to make an individual breed with it, you've got three choices; you can make the breed an "external-lnz" one (explained in the easy-edit packages) or you can remove spaces, comments etc in the usual way to fit it into the breed with a hex editor, or if you're stuck for space you can pop it in using ResHacker (which usually doesn't cause corruptions if the file is just edited the once).

Yes, you can run ResHacker at the same time as the game, and you can sometimes see the results instantly. However, it is just as likely to complain of a "sharing violation" and crash, or cause a corruption when it compiles or saves. Editing in Notepad does neither of these things.

Anyway, that's what the big deal is as regards ResHacker.