Speculating, but presumably it can be quite useful to have a ready-made and tested satellite to which you can bolt a new set of instruments at relatively short notice. Whenever something new crops up that you want to "measure" from orbit, all you have to do is build the "measuring instrument", plug it into the existing satellite bus and launch it. The only thing that needs to be designed/built is the new instrumentation; the power supply, cooling, manoeuvring, comms etc is already taken care of. If something changes, requiring you to modify your instrumentation, you just land it, make your changes and relaunch. That may well be quicker than building a new satellite from scratch, and could plausibly be cheaper too.