The Copy time to all option in the Work Schedule module copies the working timing 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.