1. Save money
Housing costs are high. Everywhere. You can save literally thousands of dollars by sharing the cost of housing and utilities.
When you live alone its really easy to become socially isolated, especially when retired or working from home. You have to have lots of extroverted energy to keep a social calendar and make plans with friends to see them. It can be exhausting. A home-mate allows for spontaneous, informal connection.
3. Share the work
Tasks. The daily, weekly and seasonal maintenance on a home is a lot of work. Sharing the tasks (or using the additional money to hire help) lessens the effort. For older people, having a younger person who can do things that were once easy might enable them to stay in the homes. Maintaining a home takes time and effort. You don’t have to do it all by yourself.
4. Live light on the Earth.
Clearly two (or more) people living under one roof are using less of the earths resources. It’s also less need for appliances, tools, kitchen equipment, all the “stuff” that is part of our modern existence.
A fifth benefit is speculative. I believe that as human beings we need to practice kindness and generosity or we shrivel up. There is something about being able to give that is enriching. When we live with others we are able to practice kindness and generosity daily.