Developers spend most of their time figuring the system out
People always struggle or avoid doing pair programming, and people always think it means that the developers' time now costs twice as much, but this is a total lie. When I worked at a place that did 100% pair programming with rotation every day, a new developer could hit the ground <i>flying</i>. Sitting next to someone who knows the system and can answer your question immediately means you get up to speed millions of times faster than screwing around by yourself. Pointed-haired bosses and accountants will never understand this. If you pair and rotate, the inexperienced person becomes experienced quickly, and it's viral if you also do rotating pairing. A team with one expert and pairing is now a team full of experts.