why do we need an"only" here?

[marymay12]

I found a strange conversation here:
Man: This speaking course takes up a lot of time, doesn't it?
woman:Only every spare moment.
I wonder why the woman's reply was not:"yes, every spare moment". I think the woman has broken the normal way of replying and the "only" is not needed at all. what's your opinion? How do you find or understand the reply?

[Raymott]

I found a strange conversation here:
Man: This speaking course takes up a lot of time, doesn't it?
woman:Only every spare moment.
I wonder why the woman's reply was not:"yes, every spare moment". I think the woman has broken the normal way of replying and the "only" is not needed at all. what's your opinion? How do you find or understand the reply?

It's ironic.