The Game Boy Advance SP was championed by Satoru Okada, who led the development of the original Game Boy Advance. When Okada first pitched to Nintendo leadership the idea of a small and thin Game Boy Advance that used a rechargeable battery, the feedback was almost entirely negative. Not deterred, Okada told one of his hardware engineers to build a conceptual model as small and thin as possible without regard to the feasibility of manufacturing the device. What resulted was the clamshell design of the Game Boy Advance SP. When Okada took this conceptual model to the same leadership group that rejected his initial pitch, they were quickly won over.

In addition to being smaller and using a rechargeable battery, the Game Boy Advance SP also addressed a key shortcoming of the original Game Boy Advance: the reflective color display that was hard to see in dark conditions.

The first Game Boy Advance SP had a frontlight that illuminated the front of the display and a reflective surface behind the screen that sent light back through the liquid crystal pixel elements to the viewer. A later revision replaced the frontlight and reflective surface with a backlight that also illuminated the liquid crystal pixel elements.

Source: Wikipedia (https://en.wikipedia.org/wiki/Game_Boy_Advance_SP)
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