Custom plugin development becomes essential when a business has a real process that off-the-shelf tools cannot shape correctly. Many projects fail because they try to bend the workflow around a generic plugin instead of building the logic around the actual need.
A good custom plugin keeps the admin simple, the frontend purposeful, and the codebase expandable. It should work like an internal engine that helps the site behave exactly as required.
That is why I treat plugin development as a strategic build layer, not just a technical add-on.