There are many reasons why penetration tests are necessary. Sometimes, a company may want to have a stronger understanding of their security footprint. Sometimes, they may have a compliance requirement that they have to meet. Either way, understanding why penetration testing is necessary will help you understand the goal of the company. Plus, it will also let you know whether you are performing an internal penetration test or an external penetration test. External penetration tests will follow the flow of an external user and see what they have access to, and what they can do with that access.
Internal penetration tests are designed to test internal systems, so typically, the penetration box will have full access to that environment, being able to test all software and systems for known vulnerabilities. Since tests have different objectives, we need to treat them differently; therefore, our tools and methodologies will be different.