I spent a few years on and off consulting working for two of the Big 4. I can’t speak for all the teams but the ones I worked with provided good value. Our job was to protect our customers from poor designs and lack of accountability. Some of the projects I worked with were very big involving dozens of suppliers. Our customers didn’t have the capability to coordinate so much work and they also lacked the know-how and expertise in many of the domains. Also many of the big engineering and tech companies out there can be very sloppy. Don’t think because they carry a recognizable name (I don’t want to name them but they are SP500) they always do good work. I have seen stuff that is really really bad, like unsafe bad. That’s where my team would step in and fill those roles on the customer side. Many of the projects I participated were also highly regulated, and that’s a whole additional layer of complexity for which you need people with experience in very particular areas. A bad design that leads to an environmental disaster or even a violation of regulations can be extremely costly.