Remote sensing data (from satellites) is unique in that it provides near global coverage from a vantage point in space. The data provides a synoptic view of the target (Earth and its atmosphere) facilitating the study of large areas of the Earth in space and time. Schellnhuber, in his article in Nature (vol. 402, C19 - C23 (1999); doi:10.1038/35011515) summarizes the need for and advantage of remote sensing observation as “New instruments are necessary, especially macroscopes which reduce, rather than magnify as microscopes do, giving Earth-system scientists an objective distance from their specimens — no longer too close for cognitive comfort.”
The satellites provide a repeat coverage of the same target area offering the possibility of using the data for easy monitoring and change detection.
The other big advantage is that a remote sensing data archive is reliable, reducing uncertainties when these data are used in models. Whereas the accuracy of the available remote sensing data is well understood and documented, you can, for example, question the reliability and accuracy of an observation made by a geologist in his field notes, especially when you are unable to contact the person.
Longer term studies on change over time in the Earth system are supported mainly through geological proxy records rather than any direct observational data. While paleo data is less spatially continuous and less detailed than some observational networks, the geological record has provided baselines and yardsticks against which to judge ongoing trends in the Earth sciences. It is a truism that knowing where we came from is a critical key to knowing where we are likely to head.
The generic answer to this question is well stated in the guide to GIS at Why Use GIS?. The reasoning stated in this guide is also valid for the use of GIS to study the Earth system.
I respond to this question again with a quote from Schellnhuber’s article in Nature (vol. 402, C19 - C23 (1999); doi:10.1038/35011515) on what he terms the digital-mimicry principle, “A more sophisticated, and less expensive, macroscope technique is simulation modeling. Here, components and processes of the original Earth system are replaced by mathematical representatives as accurate as our evolving knowledge allows. These formal chimaeras are then animated electronically, to imitate the dynamic complex of real relationships, in virtual space-time…. One significant advantage of this macroscope is that it allows a multitude of potential planetary futures to be played out, with no more a risk than a computer crash.”