Once you've got it started the engine will heat up and retain some of this heat during the day.
It needs this heat to get the diesel to burn and to start the engine, on some diesels the compression of the fuel/air mixture isn't enough to get it to explode. You need the help of the glowplugs when starting from cold after it's been standing all night.
I'm using my van all day, on a cold morning I sometimes have to wait 20 seconds or so for the glowplug light to go out before I can start it.
But during the day I don't wait at all because the heat is already there and you don't need the glowplugs.