1) More
whoosh!. Without the zig-zag
behind the digger/fortifier the water doesn't lose what little pressure it has and may well fill the void from any copious-enough water source (like a river) before the dwarf concerned gets around to escaping via the stairwell at the back. (Actually, a couple of wall-contained diagonals should be enough, on top of the diagonals from the removed barrier to the floodgate and into the temporary section. I went overboard.)
2) By letting the water through
orthagonally to the floodgate you open (as opposed to diagonally to one side) you can't now open
either adjacent floodgate without the water (albeit with the pressure taken away) leaking through the tile that you now want your digger to stand on to de-barrier the adjacent tile.
With the latter situation, especially, you probably wouldn't get anyone to stand on either adjacent floodgate's spot. Which is why I started at one side and made it a matter of working diagonally away from the yet-to-be-dug barriers. Close the open floodgate and open the one next to it and you still have 'safe ground', albeit through a route finally offset by one tile (not a big problem on anything wider than a single tile, and decreasingly so the longer it is until it doesn't matter much at all).
(Also, to add, that with floodgates you can set the floodgate control to "close" as soon as the dwarf concerned is working in that spot (or sooner, if the lever-puller isn't going to beat the floodgate-stander to his or her task position). As soon as the job is complete and the dwarf moves back away from the floodgate[1] then the floodgate is free to close and very little liquid flows through. I find this method useful when breaching magma (with magmaproof floodgate/etc, as appropriate!), for the obvious reasons, and can't remember the last time I had any problem with losing a fortification-carver.)
[1] Noting that I normally do this with
fortification-carvers, so if for some reason the dwarf has an urge to walk through the gap he's just
mined out, he's now trapped on the wrong side of the floodgate. Probably not going to happen unless a pretty good swimmer encountering some strange momentary pathing anomolies, but I'll leave this in as a caveat so you can't say I didn't see it coming, if it ever does.