Ragtek's old thread reader addon used the information in the table to display who has read a particular thread. But on 1.2.2 (err having one of my programmers test this on 1.2.3 tomorrow) after importing my previous board it seems this table is only used when a user posts. Views of threads results in no entries but a posting does.
Due some earlier keyed entries missing in my database I was afraid this was actually used like thread views was used in vB (probably Xenforo as well) to defer sometime of calculation. But there is no reason you would need to do that for posts only as that is calculated fairly easily from existing data.