Armies train in the elements of rain, snow and heat to prepare themselves for whatever weather the enemy encounters them. One mistake is limiting yourself to only training in daytime. Most confrontations happen at night with low visibility. A man may seem larger at night when fully clothed where locking up with your assailant is possible. Counters to this becomes touch and feel where knife tapping and lock flow comes into play. Moreover, training in water makes one loses his grip on things and movements becomes slow motion. Water resistance prepares the large and small muscles to sync with the entire body for common movement. It also simulates a heavier person who is pushing against you. Roadwork against the wind also attributes to this purpose. If you're out in the cold rain for hours and have to fight someone, you'll be fighting alot differently considering that you'll be dancing around puddles of water with your underwear drenched in water.
Outdoor training is essential in the fact that it takes away the comfort of the gym environment where you can easily get warmed up, have a water break or just simply stretch out and relax whenever the workout gets a little bit difficult. A scenario which keeps playing back in your head like if you would be held at gunpoint in a subway or held at knifepoint in a leisurely park at night is not only supposed to remain in your mind's eye but acted out in a proper supervised way. Being attacked in an alley or staircase or being blitzed by a mob while out for coffee or a cigarette in an al fresco cafe are some of the things that we very well take note of.
Training outside is wise, good for the soul and makes sense considering the unlikely probability of actually fighting on the ground of one’s choosing.