What every software engineer have to know

Being innovative and creative is a big advantage in a software world. Being ignorant is a curse. The basic principle is "you will never implement something what is already implemented unless...".

1. Current implementation isn't enough and can't be easily adjusted to your needs.
2. Current implementation is totally crap and you need implement it from scratch.
3. You have a lot of time or money and can do it better (don't even dream about time).
4. You can really make it better.

Anyway, to identify any of those there is one big prerequirement: you have to know about existing solutions: designs, architectures and libraries!