My thoughts (not trying to be a wise guy here) are that if you have to ask the question you might not understand or appreciate the answer.
There is only one reason ...well maybe 2, for doing a FAST thru.
- To attain some personal goal or attempt setting a new record
- Personal time constraints
Other than that, if you are going to commit yourself and your time to a thru-hike you owe to yourself (mind and body) to take your time and truly enjoy the experience.