There are many reasons why oil majors require personnel to be deemed competent in their role.  Most importantly they include:

Safety is probably the most important factor and an operator wants to ensue that all contract personnel can formally demonstrate that they are competent in safety critical activities.  The offshore energy industry is a hazardous one and lapses in safety have proven to lead to catastrophic and very costly results.

A close second to safety. Operators want to ensure all contract personnel are competent perform the activities required of them.  Failure to do this in line with procedures and policies can lead to safety incidents, breakdown, both of which can be extremely costly.

Professional Development
It is important to appreciate that undergoing assessment in the workplace, receiving constructive feedback will facilitate your own professional development.  Most contractors are required to comply with ISO9001, part of which requires them to demonstrate that they implement some form of Continuous Professional Development in their organisation.