Anybody knows why there is gba slot in ds boxes?

agb slotAGB Slot Kantschuif; AGB Slot Kantschuif 162x16mm Zwart . Merk AGB Productcode: 84376 Verpakkingsgrootte: Stuk. Vollede omschrijvingAlways checking for that slot for running nds or GBA. One of the reason, running DS ROMs even through ndsbootstap on native ds consoles have compatibility issues. DS games were meant