A: Can you hear me?
B: Yes
A: What time is it?
B: 5 o'clock
A: Thank you, goodbye!
B: Goobye!
Nothing is lost compared to:
A: Can you hear me?
B: Yes
A: Yes
A: What time is it?
[...]
Both sides need to know to "commit" to the connection.
That conversation can also go like;
A: Can you hear me?
B: Yes
A: What is the full unabridged story of War and Peace?
B: <...>
A: I am going to a take a nap for now, goodbye!
B: ... Goodbye!
A: Can you hear me? B: Yes B: What time is it? A: ...
At the point that B has replied Yes, B knows that it can hear A and that it can send to A but it doesn't know that A can hear B. As long as A makes the first move in the rest of the conversation that's fine - the next message from A confirms that B's "Yes" was received, but if A has nothing to say then B has to send it's next query and hope that A received the Yes successfully. If it didn't then B thinks the connection is established but it actually hasn't been.
In total, there are four exchanges (two questions and two answers). However, if you look closely, the second person's reply of 'Yes' already confirms that they can both hear and speak. Therefore, the second 'Can you hear me?' is unnecessary. With just three exchanges (one question and two answers), both people know that they can send and receive messages.