For MonthStart and WeekStart you don't really need to use Floor, but Floor is usually needed when you use MonthEnd or WeekEnd because the End functions give a timestamp such as 8/31/2017 23:59:59 and in order to remove the time component, you will need to use Floor to convert this timestamp to just this 8/31/2017.
You can check using Today() function
=FLOOR(NUM(MonthStart(Today()))) & Chr(10) & FLOOR(NUM(WeekStart(Today())))
If you want to see the dates, you can do this
=MonthStart(Today()) & Chr(10) & WeekStart(Today())
WeekStart will depend on what you have set in your environmental variable. But you can read about that here: