The Value of Dragonstone?

[Bollen46]

Dragonstone is a huge fortress located outside Blackwater bay that orignally was a valyrian outpost. But i´ve never understood why the Valyrians wanted that isalnd in the first place.

- I guess the Valyrians never saw Westeros as a threat since they were not united and Valyria had dragons
- it was probably not for trade reasons since King´s Landing was not founded so in that case The Arbor would have suited much better as an outpost for controll of the wine production and controlling the trade from Oldtown
- Stannis seems to hate the fortress for being cold and without fertile lands so it was probably not for colonization

Simply put: there´s nothing of value on the island to be found. And still they buildt a huge fortress that must have taken both alot of money and time to construct.
The only thing i can think of is all the obsidian that seems to be very effective against the others but the valyrians didn´t know them? And it would probably be tons of obsidian in the volcanos of Valyria anyway.

So what do you think, why did the Targaryens settle there?