The raw stats are mainly a form of aesthetic continuity with Dungeons & Dragons. Since the game uses Power by the Apocalypse math for almost all of the moves, it's the "modifier" that's more important in play.
However, note the Level Up move:
- Choose one of your stats and increase it by 1 (this may change your modifier). Changing your Constitution increases your maximum and current HP. Ability scores can’t go higher than 18.
In "normal" PBTA terms, your starting stat array is basically just +2, +1, +1, +0, +0, -1. But advancement is uneven in ways that are difficult to replicate perfectly with just the -3 to +3 structure. (Several D&D editions have a similar relationship between raising stats at level-up and their derived modifier, though they don't give you a boost at every level.)
The main explanation I'd give to confused newbies is "these are your characters' D&D stats, to give you a small impression of what they'd be like in D&D."
If you're actively hacking the game in significant ways, the "raw stats" are pretty easy to remove (World of Dungeons did, for instance). For example, you can replace the Level-Up mechanic with "+1 stat modifier every 'even' level (max +3)," if pressed, at the cost of a few quirks like making it harder for players to "round out" their low stats cheaply.
If you're just making small tweaks, however, consider this: they're already in the text and part of the character sheet, and 'faithfully' replacing the raw-stat math in all those places would actually be pretty clunky.