The e-Reader cards unlocked new levels and content from other Mario games for use in Advance 4. There was even entirely new material, like blocks that could only be broken by hammers, or see-saw platforms. Some of this stuff wasn't even available to American players, existing only on Japanese cards. The result is a kind of ultimate Mario game, with elements from across the whole series. Kind of cruel to limit it to an unwieldy card-reader setup, now that we think about it.
[Image: Super Mario Wiki]