WordPress has a plugin for almost everything, which is both its strength and one of its common problems. Installing another plugin is often the fastest way to add a feature, but it is not automatically the best long-term solution. A custom plugin makes sense when the requirement is specific enough that forcing a commercial plugin to do the job creates more complexity than it removes.
Start with the requirement, not the plugin directory
Before deciding whether something should be custom-built, define what the website actually needs to do. Is it a simple contact form, a standard event calendar or a common SEO function? Those are usually better handled by a mature, well-supported plugin. If the requirement involves a business-specific workflow, unusual permissions, integration with another system, custom data or a process that staff use every day, a custom plugin becomes more attractive.
The important question is not whether a ready-made plugin exists. It is whether it solves the real requirement cleanly without bringing along a large amount of unrelated functionality.
Signs that custom development may be the better option
Custom development is worth considering when a commercial plugin requires extensive workarounds, when several plugins would have to be chained together, when the feature needs to match an existing database or API, or when the administration workflow matters as much as the public-facing result.
It can also be appropriate when security, permissions or data ownership need tighter control. A purpose-built plugin can contain only the code required for the job, use WordPress permissions properly and avoid exposing dozens of settings that the client will never use.
Custom does not have to mean large
A useful custom plugin can be quite small. It might add a purpose-built content type, automate a repetitive task, synchronize information with another system, create a report or add a small piece of business logic. The value comes from fitting the requirement, not from the amount of code involved.
Maintenance still matters
Custom code is not maintenance-free. It should be documented, kept compatible with supported PHP and WordPress versions, tested after significant updates and written so another competent developer can understand it. Good custom development should reduce long-term complexity rather than lock the website to one person.
The practical answer
Use a well-supported commercial plugin for common problems. Use custom development when the website has a genuinely specific requirement and the off-the-shelf alternatives introduce more compromise, overhead or risk than they save.