This is important, because the site on the inside of the uterus where the placenta was is rich in blood vessels. Shrinking of this site reduces the amount of surface area that can bleed.
Involution happens quickly.
Generally, bleeding should get less and less over time, but there can be some bursts of activity for up to a month. But for the most part, most of the real bleeding should be over within a few days. Any irregular bleeding after a month is more than likely a result of the body trying to regroup hormonally. What actually happens is that its bed is separated and falls away as part of the lochia. The tissue underneath it heals and pushes it away, not unlike a scab that finally falls off.