Largely the things I photograph don't run away. I use a tripod, cable release (OK, radio remote these days) and mirror lock up. Sounds like I am thoroughly entrenched in the slow and thoughtful style of photography.
There are times though that suit the latter see and grab style of photography. Sometimes you see something wonderful and like waking from a good dream, you want to capture it before it fades.
On a practical level, it's remarkable how often a shot dissappears before you can get set up. The wind picks up after being calm for hours, almost as if mother nature didn't want you stealing the image. The light may change, a leaf falls or someone parks their mountain bike right where it spoils the image and proceeds to set up lunch. I remember once setting up to photograph a lovely cornice of snow, admittedly in a city park. Over came some kids and proceeded to knock them all down. Good thing you can't be jailed for thinking of murder. Even with still lifes I have had something slip out of position and never quite get it back right again.
There is a lot to be said for the grab it while you can, then reshoot with care and analysis if you can. It also means that whatever the subject, speed can be important - how good are you with your tools? Can you find every button on your camera quickly, with your eyes closed and with gloves on? Can you set up your tripod likewise? I don't tend to change lenses after finishing a shot, but perhaps I should replace any special purpose lens with a general zoom to increase the chances of capturing fleeting moments.