All text operations are handled by AGKs own command set. If you specify colour in the editor then you'd have to load a dozen different images to cover a dozen different colours. These font packs can be coloured, and resized with commands such as, settextcolor and settextsize. This saves a lot of device memory.
The additional shadow maps means you can setup additional effects. For example if you have a virtual light moving across the screen (easily acived by an overlaid sprite with an alpha channel) you can move the shadows with that light giving an illusionary 3D style effect. Or you can just side and place text shadows as you desire.
Next there's the legal issue behind fonts, many free font sites are only free for personal usage, if you plan to sell or make money from a game (this includes advertising) then you often need to contact the authors who'll then be able to set a price. If your game hits big time, and one of these font authors notices their font in your game you can even become the victim of legal action. I have noticed a lot of users don't read the site fine print or included read me files, they leave themselves wide open.
AGK font packs are royalty free, and completely legal to use. I won't come chasing after you demanding cash should you make your first million on the app store.
Next I have seen some of the fonts outputted from the free font editors, they are okish but the spacing is often wrong, fixed and doesn't compare when placed next to agkfp's, this is because I've expended alot of time manually creating and editing these images. If you intend to make your own fonts, up to the same standard you will have to expend the same amount of time doing it as I have.
Next I plan a few free extra upgrades for packs 1 to 3 next month, certainly before Christmas. I want to let these packs run for a month first to see what happens if all goes well I'll add some additional font maps styles.