Essentially that is what it did. For instance, you was doing an rp with someone over PM's, and you are eagerly waiting for a response from your partner. This tracking would tell you that they had opened the PM, that way you could know that it did indeed go through (as PMs tend to get lost in the internet sometimes) and has the same benefits of posting on threads, which gives you the knowledge that they may be responding soon (as you could see who was looking at the thread). Some may see it as a violation of privacy, but many people who rped through PMs on the old loved that aspect.