Monetarily, they bring in a lot of money for their publishers, and are a significant investment for a pastor. More than that, they can have an effect on the way a pastor sees his calling, and can thus affect congregations his entire life.
My pastor likes the Pulpit Commentary. I chose the New American Commentary for three reasons: I don't know Greek or Hebrew, neither of which is necessary for the NAC, it's available in Kindle format, and I liked it, based on the volume for Daniel.
That said, it's not that sort of book you read straight through. It's a little like reading a dictionary. In fact, that's a good analogy. Both are reference books full of information, and recommended to anyone who works use them as such.