Being a good uke allows nage to perform the technique without worrying about uke being injured. To condition the body. Good ukemi represents the same wisdom as that of the fisherman who through long experience can sense what the coming weather will be. Atemi, literally, means to strike the body. A simple explanation of atemi is that they are strikes.
Some people insist on more rigorous definitions such as only strikes to pressure points.
One purpose of atemi is to distract your partner, so that they focus on your hand, or their pain, rather than their grasp. This can make it easier to move. Atemi, on some interpretations, need not be an actual strike, since what matters is the effect on uke, that is, the upsetting of uke's physical and psychological balance, facilitating the application of technique. Some claim that the best way to ensure such unbalancing is to deliver a real strike, especially where there is potential for strong resistance. Some feel atemi is important in the actual accomplishment of waza rather than being independent waza in and of itself.