Reaching out to professors before applying

[mostlygoo]

This question isn't specific to philosophy but thought I'd post it here anyway. I'm only applying to masters programs this year, but thinking ahead to if/when I apply to doctoral programs, one thing I see pretty often (in the results board, and in different forums) is someone saying they'd reached out to relevant professors at a school before applying. 

I see the benefit, especially if you're going to mention a specific prof in your SOP, or if only one or two professors at a school could possibly serve as your advisor if you were accepted.  Just wondering a few things. How important is it to open some kind of dialogue like this before you apply? And how do people usually get a dialogue like this started in the first place?  I can see reaching out and just letting a prof know that I'm drawn to her work, am interested in program X and in these topics, and so on. Is it as basic as that, and letting things go (or not go) from there?  Or do folks try to actually engage philosophically and talk about their own work? (I'd feel pretty presumptuous doing that in a first email, before knowing the prof wants to engage at all.)

I'd appreciate any input, especially if you've done this in the past and were accepted into the program, or if you have any insight into how much importance adcoms place on that kind of outreach.