It is true that one will eventually develop a style, but focusing on it too soon simply leads to copying rather than mastery. Watercolor does not particularly lend itself to arbitrary decisions based on what looks good, but rather to logical sequences of steps based on what makes sense. When you focus on style without process you might achieve short-term success, but you will not consistently achieve the results you are after.
Having a process helps organize your decisions without restricting your style. This is especially true for watercolor illustration where you are dealing with multiple problems (value, composition, color relationships) that are best solved in a particular order. Planning and making decisions before you reach the detail stage will help you avoid endless correcting, which can ruin your washes and sap your painting of its vitality. With a process, each step has a reason for being and you will not feel like you are racing against the clock.
One of the best reasons to have a process is that it makes your results repeatable. Otherwise it’s like the difference between getting to your house and winning the lottery. Many of the frustrations people have with watercolor is that they have no idea why a piece turned out the way it did. A process allows you to evaluate your results and understand why they happened the way they did. You will no longer attribute your successes to “good luck.” A good process will give you confidence. Not confidence in the sense that you know what the results will be, but in the sense that you will know why they are what they are. Every painting becomes a lesson rather than a one-off event.
Finally, a process will give you the freedom to explore your style without risking everything. When you are comfortable with your method, it doesn’t matter what the subject matter is, the mood of the painting, the level of abstraction, or any other element that defines your style. You will be able to approach any problem with the confidence that your process will serve you well.
Eventually your style will emerge as a natural outgrowth of your process. It will be defined by your color palette, the speed at which you paint, the economy of your brushstrokes, and a host of other elements. This will come through repetition, practice, and confidence in your process. Ultimately process is what will help you achieve the results you are looking for in your watercolor illustration.