Some scenarios where you would write a plugin are:

• You want to execute some business logic such as updating certain fields of a record or updating related records, etc. when you create or update a CRM record.
• You want to call an external web service on certain events such as saving or updating a record.
• You want to dynamically calculate the field values when any record is opened.
• You want to automate processes such as sending e-mails to your customers on certain events in CRM.