The right for women to work wasn't instituted by Stalin's government. Women were granted equal rights by the Bolsheviks in the revolution. It is worth noting that, before the revolution, workers had longer workdays, and letting more people into the workforce allowed for less working hours.

Regarding their motivations - their goal was to bring about communism, and they believed that, to achieve this, the working class had to be united, and thus that women and men should be equal.

I do believe that in order to achieve welfare/prosperity, not all the people have to work.

A large fraction of the labour done in the present-day is excess. It is possible to meet every person's needs with less work in total. If the workload required was distributed equitably, people would have more time outside of their jobs.