To stay safe - Not only to avoid injury, but to be aware of what is going on so you’re able to find and respond to openings.
To experience the throw - Understand what the other side of the technique feels like, and observe when nage is a senior student or teacher.
To help your partner learn - Maintaining the connection will allow nage to experience and explore the technique.
To condition the body - Taking good ukemi requires a lot of work. Connection, flexibility and awareness.
Saotome Sensei says in his book "The Principles of Aikido":
"Good ukemi training will allow you to see the future truly because your vision will be based on observation and intuition, rather than an arbitrary decision made in advance of the evidence. Good ukemi represents the same wisdom as that of the fisherman who through long experience can sense what the coming weather will be."