Why do you have an accountant? Or a lawyer? It's the same thing. Corporations don't have all skills in house for a ton of things.
I was an IT consultant. A big energy company wanted to go to the AWS cloud. Their folks were too busy and had no experience. We (my consultancy company) already had the knowledge.
Consultants don't only give advice. In many cases, they also do the work. But advice is also a "product". If your in-house team does not have the knowledge or time, you hire a consultancy firm.
I was an IT consultant. A big energy company wanted to go to the AWS cloud. Their folks were too busy and had no experience. We (my consultancy company) already had the knowledge.
Consultants don't only give advice. In many cases, they also do the work. But advice is also a "product". If your in-house team does not have the knowledge or time, you hire a consultancy firm.