1. It enables them to live in lower cost of living cities. This makes them more competitive relative to outsourcing because a lower wage in a cheaper city goes farther.
2. It opens up the job pool. If you work remote, you can work at any company that takes remote workers regardless of where you live.
3. It reduces the cost of switching jobs. Many people are stuck in jobs they don't want because there are few other local opportunities and switching jobs means uprooting and moving. For a single 20-something in an apartment, that doesn't sound so bad. But once you have a partner with their own career, kids with meaningful friendships, a mortgage, etc. then moving can be extremely disruptive.
In general, more job flexibility increases the efficiency of the job market for employees.