This way, you get a longer brew time, but still get the benefits of finer paper filtering, easy cleaning, etc.
I often use this method for columbian light roasted coffees, I feel it works better for some reason. You can use less coffee, and a much coarser grind. For some coffees, and for "espresso"-ish things, it doesn't work so great though.
That's what's cool - you can tweak so much with how long you brew, temperature, grind, etc.
I add water and ground as normal, stir for a little while, then top off the water and put the plunger in place. Depending on the coffee, I then wait 0-90 seconds before depressing the plunger. The plunger seal holds the water in place with no problem.
I tend to wait the longest with South/Central Americans, and don't wait at all for Africans. Most Indonesians are somewhere around 30 seconds.