The Copy time to all option in the Work Schedule module copies the working time set on Monday to all other days. 

For example, you might have set different working hours for each day. In case you wish to maintain consistency in your work schedule, say, 9:00 am to 1:00 pm. You set this timing for Monday, and clicking on Copy Time to all will change the timings to 9:00 am to 1:00 pm for all your working days.