Service account

The service account has an API KEY associated with it to validate your request against Appian. When Appian receives the API KEY, it verifies that it belongs to a service account and the permissions related.

You don't have to use a service account in an integration, unless you are calling an appian web api. 

IN case you are calling to another system, you need to ask fgor credentials to the owner (JWT Token, API key, Basic Auth.....)

No. An integration is used to send a call to another system.

You use service accounts in web APIs for authentication when receiving calls from another system.

Hey,
To gain a comprehensive understanding and necessity of service accounts, please refer to this link: 
Need of service account in Appian

Service accounts are needed to give access to external systems into Appian i.e via a Web-API and like mentioned below the integration object is vice versa that is it is used to call other systems hence in this case an Appian service account is not required