Michael,
As others have alluded to, it's primarily done for fun or in special circumstances to test a carry yardage or see how a ball is going to react, etc. It can also be done for marketing purposes, a skill that most golden age architects aren't given enough credit for.
Here are a few images from Love Golf Design's project in Mexico. These first two are of Davis trying to qualify a back tee location on the 18th hole- notice the edge of the arroyo running down the left side of the proposed fairway.
For reference, this is a photo of the completed 18th taken during grow-in, obviously from a different angle above and to the right the teeing area.
This image is of Davis' brother Mark hitting from the first landing area on the par-5 17th. He was checking to see if the elevated green is potentially reachable in two without the help of the prevailing wind- it is.
In most cases, the designer doesn't have to hit that many shots because the owner or guys on the construction crew are doing it for him. If you want to know how certain holes are going to play well before a course opens, these are the guys to talk to.
John