When Should I Use Apex?

Salesforce provides the ability to customize the pre-built applications to fit organization. But, the organization may have complex business processes that are unsupported by the existing functionality. So apex should be used to implement the complex business functionality. Theses includes APEX, Visualforce and the SOAP API.

Apex
Use Apex if you want to:
-Create Web services with integrating external system.
-Create email services. For example, automatically creates contact records based on contact information in messages.
-Perform complex validation over multiple salesforce objects.
-Create complex business processes that are not supported by workflow functionality or flows.
-Create custom transactional logic (logic that occurs over the entire transaction, not just with a single record or object) like using the Database methods to insert, update or delete the records.
-Attach custom logic to another operation, such as saving a record that causes trigger to fire, an action on Visualforce page, or from SOAP API.

Visualforce
Visualforce consists of a tag-based markup language which gives developers a powerful way of building applications and customizing user interface. With Visualforce you can:
-Create wizards and other multi-step processes.
-Create custom flow control through an application.
-Define navigation patterns and data-specific rules for optimal, efficient application interaction.

SOAP API
-Use standard SOAP API calls if you want to add functionality to a composite application that processes only one type of record at a time and does not require any transactional control (setting a Savepoint or rolling back changes).
-Use SOAP API to create, retrieve, update or delete records, such as accounts, leads, and custom objects
-SOAP API also allows you to maintain passwords, perform searches, and much more.