A model system is one that has features which make it a particularly good system in which to address whatever question the investigator has posed. That can be for a couple of reasons. One is that the system has features that make it easy to collect needed data. For instance, that’s why small organisms with short generation times are a good model system for long-term studies of population dynamics. Another is that there’s lots of relevant background knowledge, on which you can build in order to ask questions that couldn’t be asked without that background knowledge. For instance, Drosophila melanogaster is a model system for modern-day genetics and genomics not just because they’re easily and quickly reared in large numbers, but because Thomas Hunt Morgan used them to study inheritance over a century ago. Morgan’s work was built on by others, who couldn’t have done their work had Morgan not done his. And others built on that work to do work that they couldn’t otherwise have done, and so on. Perhaps the most powerful model systems are those that are good for asking many different sorts of questions, so that the answers to those questions can be linked together.