Break free from feeling limited: Client projects aren't always things you can just solve via the JED
Break free from feeling overwhelmed: Even if they are, you don't always want to install an entire component to provide functionality that you could do with 30 lines of your own code
Other components probably need to be maintained going forward
Other components may be bulky and burden your site with a lot of unwanted code
Other components may have vulnerabilities
Other components may slow down your site, add their own overhead or load outside frameworks, conflict w/ your existing template or extensions, etc.
Develop safe, quality components: Of course, many components are well developed and provide great value to clients -- and you might want to contribute to the JED yourself so that people can benefit from your solution.
Clients shop around; Be the one who says "Yes, I Can!" New clients routinely tell me they couldn't find a developer who could realize their vision.
Notes
These are the slides from Jim Dee's session "Leveraging the JDatabase Class," as presented at the 2016 Joomla World Conference in Vancouver, B.C., on 11-13-16. I've updated them with a few minor corrections / additional notes. Thanks!
This page: https://www.arraywebdevelopment.com/courses/jdatabase/intro01.php