I am very meticulous about trying to get all the crud out of my gravel - there always seems to be so much of it. If I tried to get it all using the regular syphon vac, I'd be taking out ALL of my water before I was happy with how well I'd cleaned it.
So as it stands, on my weekly/week and a half clean, I usually siphon 50% of my water out while I clean. If it still looks quite dirty to me, THEN I whip out my little battery powered vac with a dirt collection bag which pushes the water through the bag and cleans it further but without the need to remove any more water - there's just no need to remove that much, and living somewhere with regular water restrictions, I can't justify wasting all that water either.
It's not long enough to use when my tank is full ( mine can only be submerged 14 inches or so, my tank is 18 high) so I get around this by removing the amount of water I plan to change so the level is low enough and then I can vac and clean to my heart's content with the other one.
I agree with posters who say you can't use one of these and not a regular siphon vac too. Both, or not at all. The bag one doesn't do nearly as good a job as the other, but like I said, I only use it when I want extra cleaning without wasting all that water.
Hope that made sense. They are worth getting if your reasons are similar to mine.