The default value of a “unit” in 3D is a meter, so 9.8 is an approximation of real-world units.
On the other hand, in 2D, units are pixels, so it can vary widely depending on the size of your sprites. The units will be in pixels/sec^2, but 98 won’t really have much relation to reality unless you size your sprites in such a way that 10 pixels == 1 meter. In practice, especially in 2D games, a higher gravity value results in more responsive gameplay.
Thanks sparkart & kidscancode.
Knowing it is in pixel/sec^2 is useful as it gives a good baseline and takes away some of the “finger in the air” guess work.