Spotting problem with your plan and being able to foresee difficulties are both essential skills for a project manager, but I would argue that while the first one can be acquired by anyone willing to put in the needed effort, the second skill is much harder to develop. This is why I believe in the importance of managing a project in a field you master.
Everyone does not share this view. Some people indeed hold the opinion that project management is a “standalone” skill: once you master the mechanics, you can apply it to any context. In the same vein, it could be argued that even a project manager with a lot of experience in a given field cannot possibly hope to master every single aspect of the project he will be working on. Why bother at all with choosing someone with field experience, then? This perception of project management does have its appeal, but it forgoes some of the most compelling advantages offered by choosing a project manager well versed in a given field.
The first one is obvious: when you know what you’re working with, you also know who to turn to when in need. Good data is essential when the time to take a decision comes, and that data can only be obtained by asking the right questions to the right members. Field knowledge is definitely a big plus in this case.
There’s also the fact that without mastering every skill, a project manager with knowledge of a given field still usually has a good idea of what every member of his team does. This is invaluable when evaluating the impact of a decision. Unforeseen consequences can be very damageable to a project’s progress; the more you know about your field, the more you can plan ahead.
Finally, a lack of project management skills is simply easier to remedy than a lack of field knowledge. Between coursework, mentoring and following commonly accepted best practices, the options are numerous and accessible to anyone willing to learn.
Combining field knowledge with project management skills invariably leads to making better decisions. Which is ultimately what being a good project manager is all about.