I do it because of the chance of, lets hypothetically....."You" call and ask me to call you back before 6, so I call you at 5:45 when I am leaving work.  Your wife meanwhile, shows up early and you don't take the call saying something like..."don't know who that was"........  Well, I don't want her calling me back to SEE who "THAT" was...lol.

Also, if my cell phone is low on battery power, or I want to use my landline rather than pay a dime a minute, I will block that number, but pretty much the main reason is the first scenario.

Hope that helps!

I just remove my name, my number is there for all to see. Heather