The shape of the keycaps makes particular typing styles / finger motions more or less convenient.
For most people, I suspect (I haven’t done a wide-scale formal study, this is just my observations; YMMV) the main finger motion use to press a key is flexion at the proximal finger joint (the metacarpo–phalangeal or MCP joint) where the finger meets the palm. Likewise, the main motions used to locate keys are flexion/extension of the further finger joints, the interphalangeal or IP joints, combined with some rotation of the wrist and movement of the whole forearm, depending on the key.
If the keycaps are all flat across the top, then to reach a further key (e.g. on the number row) requires either moving the hand or stretching the finger out forward, including a bit of flexion at the MCP joint. These are some combination of slightly slow and/or uncomfortable: if you move the whole hand around, then you need to move it back to strike the next key, and in extreme cases some key sequences become difficult to type; or if you stretch the finger out (still on a flat keyboard), the flexion in the MCP joint reduces the further range of motion in that joint, and the action of actually pressing the key by flexing the joint becomes weaker.
If instead the keys have a bit of a vertical step between rows, just extending the IP joints will place your finger closer to the key top, requiring little to no flexion in the MCP joint to reach the key top, and less whole-hand reaching.
This would all be easier to understand with a picture or with a physical demonstration using some keys and a finger, sorry. :-)
(I gave a lecture about this subject at a bay area keyboard meetup, and should really someday get around to writing up an extensive website about my thoughts on keyboard ergonomics complete with many diagrams and videos. Not today though.)
For anyone trying to learn about this for themselves, I recommend trying to get hold of some keycaps of different profiles, and directly compare some alternatives. They can be ****ty printed ABS ones, you can mix and match wildly different keycap profiles, or you can adjust the profile by sticking bits of modeling clay on the tops, etc., doesn’t have to be production quality. Pay careful attention to what motions your hands and fingers make as you type, and think about what muscles are involved, etc. Read a book on hand kinesiology if you feel especially motivated. If you have the equipment for it, try taking some high-speed videos.