Breaks the Monotony
Long runs are notorious for being monotonous, especially if you live where you have to do a lot of out-n-back or repetitive loops. Having others around you can do wonders in breaking that monotony, only if it's knowing that others are experiencing the same thing you are.
Makes You Faster
It's very easy on your own to skip that weekly hill workout or that weekly tempo run. Missing multiple speed workouts can really hamper your ability to increase your pace or even make your prescribed race pace. The accountability factor of the group will help you make most if not all of those workouts, making you stronger and faster. Also, one of the best ways to get faster is to run with someone who is slightly faster than you. In a group you'll have the opportunity to "up the ante" and challenge yourself by running with others who may be slightly faster than you. Or you may be the one to help challenge a slightly slower runner.
Doing long runs in isolated areas or nighttime runs by yourself can be a safety issue. Also, if you're experiencing a medical issue (asthma attack, heat exhaustion, etc.) or stumble and fall and get injured, there will be others nearby to take action.
Makes the Training Fun
Even if you're not a talker, just begin around others, hearing their life stories, and getting to know more about each other can be very entertaining. Many group runs my abs have hurt afterward from laughing so hard. So, it's a good core workout too.
Marathon training takes about 4 months. That's a lot of time with your fellow group members. Time and time again, I've seen lasting friendships form.race.