Perhaps if you shared more about what you were looking to do the community could point you in the right direction.
Without knowing what your objective is or how the WIM will be used, I'll attempt to give you a 'thirty-thousand foot view' response.
Capturing an image: You have a running instance of Windows. Maybe it has all the software you need and setup just the way you want. Maybe it is configured a certain way. You want to be able to get back to this specific point in time. You capture an image of Windows as-is.
Modifying an image offline: You captured an image 3 months ago and you just realized you need to change it. Maybe you're adding/removing a feature or driver. Maybe you're patching it. Maybe you're adding, removing or modifying a file. Without booting that WIM (aka offline), you can make your change(s) via DISM.
Generally speaking, IT pros do both:
Build the reference machine
- Install Windows
- Configure the OS
- Patch the OS
- Install & configure software
- When everything is to their liking, they sysprep the machine and capture an image of it. (This is the only Microsoft supported method for preparing a reference image for deployment. Certain applications may require you to perform other steps prior to sysprep'ing the machine; others simply aren't made to handle that but they're few and far between.)
- They now have a WIM that can be deployed to 5, 10, 150,000+ machines.
Modifying the image offline
- The WIM was captured in April of 2015.
- The Windows Update portion of the imaging process takes take 60 minutes due to the sheer number of patches.
- The WIM can be serviced offline to add those patches
- Once updated, the Windows Update portion of the imaging process is now only 15 minutes
If you're building a thick reference image with loads of applications, fully patched and/or with a specific configuration, build it online.
If you're building an image where you're ONLY enabling/disabling features or adding/removing specific packages, files, drivers or updates, I see no harm in doing that offline.
Most do the former as there are limitations to offline servicing:
- You can't install .MSI's and .EXE's offline.
- DISM offline can handle .CAB files, .MSU files, .INF files or a folder containing one or all of the previous file types.
- Some patches/updates cannot be applied offline.
- Some patched/updates must be installed exclusively.
- Some patches/updates have prerequisites that cannot be applied offline.
- When adding patches offline, the patches are queued (in an Uninstall Pending or Install Pending state) and only get applied during the first boot.
- DISM online is identical to DISM offline except it cannot handle .MSU files
- If you need to install .MSU files online, use WUSA.
- I'm sure there are others.
Because of that, its easier to do everything online, build your base then maintain it offline (within reason) moving forward.