Out of the box, Lightwell comes with 5 native fonts: Arial, Courier New, Georgia, Helvetica, and Times New Roman.
While these tend to be staples, designers and developers tend to use a much wider array of font styles. In order to address this when importing from Sketch, Lightwell attempts to locate any custom fonts that aren't in the core 5, and copy these into the project for use.
For example, I have a Sketch text layer that's using a font family not found in Lightwell (Andale Mono):
After importing into Lightwell, I can now select Andale Mono as a Typeface option within the project I imported into:
It's important to note that the custom font being used in Sketch must exist somewhere on the user's computer. Lightwell will browse a number of folders that usually contain font family files, and attempt to copy the font into the Lightwell project. If it is not found, Lightwell will default to using one of the built-in font families instead.
Once custom fonts have been imported, it's useful as you are able to use that font family on any subsequent text layers you may create in Lightwell, or for any other text layers that had been imported.