There are a few objectives:

Design:

• Provide a better mobile experience. The current controls appear too small on mobile.
• Be consistent with our design guidelines and remain consistent as they evolve.
• Don't introduce new icons: Maps currently uses Mapbox icons that are different from OOUI icons (like the + and - icons)

Engineering:

• Don't reinvent the wheel, use our homemade UI library controls instead of redesigning Leaflet controls