This function returns the size of the (outbound) packet history buffer in the current session. It is a measure of how well the network connection is working; if it is monotonically increasing over a longer period of time (say, tens of seconds), that means data is not getting through to the other end, or that acknowledgments of packets are not getting through in the other direction.
An application can use the value returned by this function to make a decision to drop the connection altogether, if it exceeds some specific threshold. Such a threshold should be configurable.