At higher levels ukemi serves several functions -
It protects uke
It protects nage
It pushes nage to higher level practice
Under multiple attacks the danger is very real for uke, they must clear the space when thrown and be aware of needing to take ukemi from unexpected sources.
At higher levels ukemi serves to protect nage as well. Uke must be able to apply and adjust their energy so as not to injure nage and/or frustrate them with failure.
Uke can also push nage to make them work harder at their technique. In this way, training in Aikido progressively approaches realism. Watch the variety of ukes in this sandan grading. Some push nage Sid harder than others: