Your Email is Your Electronic Filing Cabinet
Think of your email account with its inbox, folders, contacts and the such in the same way as you would your filing cabinet. You wouldn't leave all your paperwork sitting on your desk in no order whatsoever, the same approach should be taken with regards to your inbox.
Each persons' filing system will be different as each of our jobs, needs and goals are different. Some people do better filing all their documents by date, others by the name of the people who sent the emails, others by project, others by client, etc. The good news is that it's very easy to build a system within your email account and to change it as you move forward. You can either do this build yourself, or hire someone to build your system for you, and maintain it for you as well.
For myself, I work with both electronic files and paper files. Despite our best efforts, some work still is paper-based, accounting in particular. Many people scan all their paperwork to electronic files, I am not one of those people but it works beautifully for those who do that.
The less time you spend hunting for files, the more time you can spend on other things, more important things.