Waiting on the host side is inaccurate and leads to desyncs in the event of the sink missing a deadline that require stalls to fix. By waiting for the sink to have space before even starting rendering such desyncs can be avoided.
This avoids the need to stall if the host sink sporadically misses the deadline, in such a case the previous implementation would report them samples as being played on-time, causing the guest to send more samples and leading to a gradual buildup.
Billy Laws