Why match transistors?

Alright, I know that matching them is obviously the better thing to do, but my next project uses quite a few small signal transistors and I was wondering if I really have to match them all. What's the difference really between matching them and just selecting at random? Basically I'm a lazy guy and all I want to know is will **** break if I don't match them heh...
You can use BC847BS and BC857BS. They are very cheap but small. Works like separate BC847 and the matching is fair, remember the price! The big advantage is temperature matching. I have tested them in my monster overkill phono amp and they worked very well.

To answer your question: When you use differential stages and currrent mirrors it can be a good idea to match or use matched transistors but I would say it's more important to keep the temperature equal. I would also say it's more important in diff stages to match.