My company, directly following full FDA approval of the vaccine, recently mandated this. They argue that it's the "right thing to do for society", but likely are also driven by knowing the vaccines have been proven to reduce, limit and in many cases prevent the symptomatic effects of Covid. Yes, even vaccinated people are getting it while carrying a high enough viral load (and with Delta + similar load to non-vaccinated individuals) to transmit the virus to others. However, they have seen that employees will have less downtime because vaccinated employees, on the balance, will have significantly less impact and be able to return to working (even remotely) more quickly.
Additionally, for parents if you or your kids pop a positive test then you are going to be without childcare for 10 days which again limits your ability to work from home.
Lastly, they have significant financial interest (from insurance perspective) to keeping medical costs down. That's why they give cash bonuses for health life choices generally (smoking cessation, healthy sleep habits, working out, passing physicals, etc).
