When I wrote The Fighting Edge between 1996 and 98 there was only one kind of practical self-defense guide on the market, those written by bar bouncers like MacYoung, Thompson, and Peyton Quin—the latter of whom was savagely attacked and nearly killed along with his friend while leaving a bar. Not a regular bar-goer, when it came time for me to write such a book, I accessed bars in the worst areas—and some better—to meet up with violent people and interview them. These facts should tell you something about bars.
I was operating under the assumption that those who relaxed in what is actually a dangerous environment might be assumed to live, travel and work in likewise perilous circumstaces. I did not want to write a book on bar-fighting, which can easily be avoided by not going to a bar, but a book on urban survival, with the urban matrix being dotted with many refuges from apparently worse circumstances that surround them. In a rural setting the bar is where you find danger as a young asshole. In the urban setting it is called The Watering Hole, a refuge from the violence outside, but a refuge that often sets the bargoer up for violence of a sort worse than recreational redneck bar-fighting. In the city a bar is a place to hide from the evil intentions of cops and criminals. I have used it for both.
The Watering Hole is aptly named as it is a place where hunters observe their prey coming and going. And, just as a giraffe is more vulnerable as he drinks, so is a man. Cops and criminals stakeout bars like lions and leopards stalk watering holes. Coming and going from any location has its risks:
1. Shopping venues pose an equal danger as those going in have more cash and those leaving are encumbered.
2. Relaxation has its risks, with predators striking the relaxed victim. A man returning home, the worries of the day behind him, is more apt to be hit than the man emerging in the morning, alert to his coming toil. Conversely, the man pumped up and pissed off about his day at work, heading to the bardoor is not often waylayed, rather he is mugged once he has finally relaxed with the help of booze and is on his slumberous way home.
3. Bouncers, doormen, barmaids and dancers are specifically hunted as they leave the establishment, mostly closer to their vehicle than to the door. 
The bar itself poses five specific risks:
1. the patrons within
2. those who might seek you at the bar, seated, relaxed, back to the walkway
3. unique lighting that is neither compatible with daylight or night and causes some disorientation when leaving. This neutral, often time-of-day-nonspecific lighting, is part of the bar's charm, the ambiance of its timeless function as a place to relax.
4. Noise compression in the bar dulls the senses for the night.
5. Your combat readiness is reduced from socializing, sitting, standing, leaning and most of all, drinking.
1. Gathering information, which has been my primary reason, one now reduced to fishing for odd quotes and character sketches which can be had elsewhere.
2. You rent a room and don't have any common space to read, watch TV, etc. This is what I use the bar for during the day. If I'm too tired to stay awake reading in bed, I take the book to the bar, stand amid the dull noise and make my notes. The reading for Of Lions and Men, including an annotation of the Iliad [far short of Aristotle's I'm afraid]
3. Meeting a person
4. Finding a person
5. Eating. In many city locations the only decent food is in bars, with carry outs often staked out as mugging sites. You can eat in the bar instead of trying to fight off Jamal and Tyrone while holding your pizza box.
6. To get away from your woman.
7. To relax before getting home to the nagging that awaits.
8. To listen or engage in group discussions, a common past time in some city bars.
1. To meet a woman
2. To get drunk
3. To flex, strut, pose or otherwise make a show of your masculine prowess 
1. Convergence predation is discussed in When You're Food.
2. This factors heavily in your selection of drinking companions.