Knowledge, skills and good equipment
In order to be a good and professional sound engineer, one should not only have qualifications and skills, but also work on proven equipment. It can be different. Renting a TV or other devices for a party, conference or concert is a solution that works, but only if they are high-quality devices with excellent technical parameters. Even the best sound engineer will not be able to create an appropriate sound system in a situation where he has poor quality equipment at his disposal. At the stage of education, sound engineers learn on really professional devices and they should also have such in their daily work. The best solution is to have your own company and equipment, so you can be sure that the services provided will be of a really high standard.