There's an old line about GDP that that if a house gets torn down and rebuilt, that adds to GDP. If someone pays people to dig a hole, for no good reason, that adds to GDP. If you build a bunch of weapons that are never used for anything... that adds to GDP.

At to that, this example, from Louisiana post Katrina (speaking about increased tax revenues):

But the biggest surge by far has been in sales taxes, as hurricane victims have used federal aid, insurance proceeds and their savings to replace items as disparate as socks and S.U.V.'s. Officials forecast that the state will end up with almost $500 million more in sales tax revenue than they expected before the storms hit.

Is Louisiana richer or better off than it was before Katrina? Of course not. But there was more economic activity than there was before Katrina, because of rebuilding.

GDP measures the amount of activity in an economy which is part of the "money economy". It doesn't tell you if you're spending it to get ahead, on anything productive, or if you're spending all that money digging a hole in the ground.