It's a great way to see a far away place from a local's perspective. Usually, the local Do organizers chose excellent restaurants, bars, activities, etc. The UK's Jenbel comes to mind as exceptionally talented at this.
It's the kind of local know-how you could spend (waste?) a lot of time scouring guide books for.
There's also some great comraderie to be had when you're sharing the flights with other FTers.
Even the "mini-Do's" (the "im at ____ tonight, anyone up for dinner/drinks?")..these are great ways to learn about great places to eat if you're stuck at an airport. Lucky9867coins and his Hola! Do's come to mind-- as does a fantastic brunch in FRA, hosted bey cathanna & flygva, and a great Greek place near LAX a couple of local FTers introduced me to.