Aloha Kevin. Good to see you around.
That's one of those simple questions that asks a lot.
In general, amp sims attempt to mimic the characteristics of various head/cabinet combinations or units that have a particular sound.
Many have options for mic placement in front of the speaker(s) as well as stomp box effects.
To an extent, your Digitech pedal does some of the same things. I have an older RP80 that I like to use in front of an amp sim. I have used it straight in, but often it doesn't quite get there, soundwise.
The amp sims allow you to play with cabinets, etc, to flesh out the sound.
I used to really like FreeAmp2, but sadly, it doesn't work with the newer Cakewalk products. It has something to do with the underlying code. I never got into the FreAmp3 setup, and started looking in other directions. You might want to try it, just to see if it suits you.
I like Nomad Factory Rock Amp Legends, but it's a little pricey right now. I got it during a crazy cheap special. You might want to keep an eye on it, in case a deal comes up.
I've just spent a little time with ACME BarGig Shred, and I really like what it brings to the table.
Definitely check it out. I believe it may offer what you are looking for.
You can choose your cabinet and play with room acoustics.
Most importantly, it doesn't seem to have the problem with that fizziness that can appear in many sims and is noticeable with some presets on my RP.
Hope that helps.