As film speed increases, so does grain size. Very fast films have large grain size. Some photographers use it to acheive a grainy look to their images for creative purposes. Most photographers use it only when a slower film won't do the job. You can meter however you like.
I can't really think of a reason to use it during the day, unless you wanted a really grainy picture, but then you run into the problem of not being able to stop down enough to even get a picture. You would probably need a neutral ensity filter. You might use ISO 800 film to shoot sports and get really fast shutter speeds in the range of 1/2000 to 1/4000.
What mode you use on your camera doesn't matter. Shooting any medium, be it film or digital is the same. You have to meter your light and pick the best exposure for the scene.
I used to shoot 3200 speed B+W, and 1600 slide film for indoor sporting events (arena lighting, no flash). I have occasionally used 1600 slide for foreground / sunset background shots. I carry a set of 4 ND filters but almost always finish off a roll, and then use the ISO film I think is best for the shot. I am not one that worrys about wasting film.
The only time I can think of using a fast film in daylight would be if your in a canyon / forest or something and just about everything is in shadow. Or if you want the grain effect.
I wouldn't use it unless you need all that speed. I used to shoot Fuji Press 800 when I first started so I didn't have to worry about camera shake. I didn't have the patience for a tripod. I'm glad I used it, as it helped me learn, but I do have one shot from that period that I still like, and I wish it wasn't so grainy.