Traditionally, the uke is in the role as the teacher. What follows from there is obvious.... What kind of teacher would you like to be? When you can begin to ask that question to yourself, you can begin to understand one important part of that role. At another level, the uke is practicing the process of receiving the intent, energy and force from the nage. What you do with what you receive is also important in developing the ability to better connect with your partner and use that connection in a constructive manner.
Traditionally it really depended on what was being done. For sure in Kenjustu uke was more often than not the senior but again it depended on what was being taught. In jujutsu training (which includes aikido) you often spent alot of time receiving the technique before you started performing it yourself.