It's a way of you telling them you're not going to tell them if you'll attend or not.

I see it a bit differently, I see it as 'I have seen this invitation and know this call is happening, but I have other priorities at that time. I will attend if I can, but I don't think my participation is important enough to reschedule', and it gives the organiser an opportunity to say 'actually, we will need you on this call, can you suggest a time when you could definitely make it?'