It's not obvious. Most of the people on this site, if they sold their website or whatever project for 100.000, will blow at least 20.000 on pointless stuff.

There is nothing wrong with blowing 20% of whatever you make on pointless stuff (though 10% would probably be more prudent), key is not blowing the other 80%.

I think it is pretty obvious; it's just not easy to restrain yourself from the temptation of getting a new toy, and it's too easy to rationalize why it's not necessary to save right now.

The fact that it's obvious but not easy does not necessarily mean that it's not worth reiterating periodically. Quite the opposite.

Another good example of something that's obvious and hard to do is "Make something people want" :-)