Keeping the best people at a company over time is usually driven by perceived upside, cultural fit, and enjoyment of the challenges they solve. A regular program including happy hours can help to strengthen the informal bonds that keep someone happy with the situation they have.
Reminder of Purpose: A strong communication program that shares the company's progress can help employees be proud of their company's contribution to the world and their part in making that happen. It's refreshing to pull back from the day to day sometimes to remember the bigger picture and celebrate how work is contributing to a larger societal mission.
Strengthen Informal Ties: The game-changing work often involves more than the work formally assigned. Whether it's building relationships with leadership in mentorship programs, punching up via leadership of committees or working groups, or helping colleagues in other functions to solve critical problems, it's important to develop a network within a company and be well regarded by more than just a manager. Happy hours and other company functions can help to keep those connections alive and foster collaboration and innovation across the different groups in the organization...which can often help people to channel their passions and develop internally the exciting initiatives that will create a sense of ownership and keep them excited about their job.