Default limbs are really just one of many ways to rig shape layers to an IK framework, with custom limbs, Limber becomes even more powerful. Through Limber's Copy and Paste functions, there is a simple way for users to access and efficiently handle many other rigs.
Most animators are not as experienced with complex expressions, math and shape layers as we are; so we ship Limber with a library of custom limbs that we've designed and thoroughly tested for you. We update the limb library with new limbs and optimizations fairly regularly, and include it with new updates to Limber (subscribe to email updates from aescripts to be notified of new versions).
The limb library is a standard .aep project file. It's organized into a series of separate comps, one for each limb, with notes that explain how they work. You don't have to use only the limb library file we give you - you could also create your own library file with limbs you've customized yourself. How you organise the files is up to you. But we recommend that you keep your limb library in a folder where you'll always be able to access it, perhaps on a cloud service like Dropbox or iCloud.
The Import Library button simply provides you with a quick way of importing the library. It will store the directory path to the file, separate to After Effects' standard Import process, and save you from having to navigate to the directory every time you want to import it. The Import Library dialog provides you with buttons to change the stored directory path, reveal the location in your OS, and select from several library files.
Once you've imported the library, you can browse the custom limbs, and copy and paste them into your characters existing limbs. Remember you can Alt-paste to preserve the Length, Size, Color and other options in the Limber effect of existing limbs.