David Allen walks you through the essence on Capturing everything that has your attention.
Why capture?
Using your head as a system to remember the things you should do something about is not efficient. Your brain works best if it has little that pulls on your attention. If you have too much to think about you will, in addition to becoming stressed, lower your level of problem solving capability and your level of creativity.
Why Capture on paper?
In these hectic and digital times we live in it's often a lot faster to capture on paper.
Compared to pulling out your mobile, unlocking it, finding the right app and long-press for Add task and then start writing, it's a lot faster on paper.
Pull out your wallet, pull out the pen and start writing.
No distractions
Capture freely
Capture fast
Sometimes old school is best