I think the spoilers allow the plane to lose height on landing approach without increasing speed
spoilers create an extra-drag that can both help decelerating the a/c or increasing the vertical speed without any further "forward" speed increase.
some pilots apply them without any warning to pax, causing alarm down the back as this part of the plane shakes, Is it the disturbed air from the main wings hitting the tailwings ? several times Ive had to reasure people around me that its only the spoilers and all is OK
It's true that, as for any commercial flight, you fly people not planes ... at that purpose it would be appropriate, whenever possible, to properly inform passengers about what's gonna happen. That said, if you're goin' to use speedbrakes to increase your VS (eg. very high on vert. prof), thus assuming attitudes that a pax may find unusual, here the announcement could be of use. Anyway, should you be goin' to deploy speed brakes just to slow the a/c down, no announcement would be required as your pax's couldn't care less about what you're doing to fly the a/c.
I'd suggest everyone to have a look on this .pdf:
http://www.smartcockpit.com/data/pdf...Passengers.pdf
Happy Landings,
fredgrav