If you don't study the markets, you lower your chances of selling anything because you don't know what publishers want. If you send in something that's outdated or no longer of interest, you frustrate editors. Do it often enough and you create a negative reputation among editors (and they do talk with each other).
As you study what's out there, you can ask yourself, "To what does that lead?" You can learn to anticipate what the public will read next. For example, I've been suggesting for five years that books for retiring baby boomers will be a big thing. So far I haven't seen many books on the topic, but they're definitely on the way.
Studying the markets is more than selling; it's staying abreast about what goes on in the world. We figure out the felt-needs of people, sometimes before they're aware.
We study the markets because we're professionals.
Professionals are always on the learning curve.