Two reasons in my experience: first, reproducability gets orders of magnitude more complicated as you grow. One person can do something over and over with no documentation, a few people can learn from them, but once you get to 10 - 20 staff in a role and you have some churn, you need good documentation, training, and QA since your risk for errors goes up no matter how good the documentation and training is.

Second, as others have mentioned is coordination. Now introducing a new process, feature, or whatever to clients or internally means creating the requirements, the documentation, working with sales or client services on timing and release, working with QA on test scenarios, working with training to get it into training programs, etc.