Convenience is probably the reason. If only USCIS built a good app that could help tracking one's cases and get immediate notification when case is approved. I'm aware of text message / email alerts for updates to the case and uses them too.
Another reason for using websites for tracking progress of hundreds of cases is to know which cases USCIS is actively working on. The official processing times are not transparent or detailed enough. But third party apps give a better insight. For example, when my I-751 was pending I checked the status of about 10000 neighboring cases around mine filed within a month before and a month after me. I kept watching it few times a week. One moment USCIS started touching neighboring cases - issuing RFEs, scheduling interviews, approving and denying ROC petitions. That gave me an idea that my case is going to be processed within a month or so. Soon after tracker app showed cases being looked at, VJ folks in relevant month threads started posting updates about their cases. So it may be useful to some people.