Beyond the usual suite of rankings and game database, it also had a string a small geekish tools which would give various statistical insights: given a field of players, what would be someone's likely performance, what would be the distribution of points one would get, what would be the % chance of podiums, etc...
It would also calculate your tournament performance (rather than adding your results one by one in the FIde calculator).
It also had some bizarre player analytics (like propensity to exchange queens and various odd bits) that can't be found elsewhere (and are probably completely useless KPIs).
Completely useless, and hence, utterly indispensable for anyone with some likings to statistics