Costa del Sol Villa Side Quest
In the vacation town of Costa del Sol you're able to buy one of the buildings for a whopping 300,000 Gil! After purchasing this building you'll be able to stay at the Inn for free whenever you want but that's about it. There is no other bonus you get for purchasing this villa - even the treasures in the basement can be acquired without owning the building.
Depending on what version of the game you're playing there may be a Trophy or Achievement for purchasing this building... In which case I guess that's a reason to purchase it if you can't think of any others.
For those of you who don't know about the mastered All Materia trick, you can make millions of Gil in less than 30 minutes simply by mastering and selling All Materia. For more information about making Gil by mastering All Materia follow the link provided.