As an associate, odds are strong that you’re leaving eventually. Maybe you’ll go to a different firm. Maybe you’ll leave the law entirely or head into government lawyer role. Maybe you’ll start your own business. Some will, when knowingly turning their work into a way to grow themselves, see this as taking time or energy away from their employer.
When you get better, when you grow, the law firm gets better and grows. You don’t exist outside of the ecosystem of the firm. You are a piece of the building. Your growth makes the firm bigger, stronger, more capable, and more valuable.