Marketing is all about getting on front of the right eyeballs, but once you're there, then what? Do you really want to be vague, or worse, misleading, in what your customer should expect when working with you?
If you are vague, then they will set their own expectations, which likely will not be communicated to you, and more often than not, will result in a bad customer experience and a bad review. If you're being misleading, there are deeper issues that need to be remedied before moving forward.
By telling customers what to expect, you are managing their expectations and thus you can predict what they will expect from you (within reason); this can serve to be a huge time saver and ease the load on your front line and/or customer service staff. Why WOULDN'T you do this?