As your website matures and your business grows you always need new features and options to reflect new features, options, changes in company on your website. Sometimes it's only content update, but when you have real changes in your business, you always need some functionality changes. There is always something you want to improve or something that wasn't done properly before. In any case, customizations are the most wanted jobs for existing websites.
Advance your website functionality with new features and options.
Integrate 3rd party extensions and services.
Optimize website performance and increase security.
SEO optimization always requires functional changes to achieve best results.
Fix old bugs or change functional process flow entirely.
Replace old hacks with modern, effective solutions.