Assuming a team of all at least proficient developers, one who's living a peaceful life outside of work is going to be put into the first category, and people with more chaos in their lives (not by their choice) will fall into the latter category. As their life situation changes they have a bigger chance to lose context, forget who's who or miss out on word of mouth; or conversely, become a sponge for company culture and leave the office with energy for fulfilling projects outside of work and get involved with, for example, open source where they gain insight that they take into work later, for example perspectives on business specific knowledge or some analogues.