Hello Vigneshwaran,
the simple concept is when you are waiting for any window or any element you should use wait stage since you are not sure how much time it will take to show up that window or element. it means its not static. so mostly in object you should use wait stage since you are interacting with application.
in process you don't have wait stage so you have to use sleep.
Sleep is static wait if you use it BP has to sleep for that interval but when you use wait stage with condition.as soon as the condition matched it will break the interval.
