I've been a consultant and I've been a dev who's worked with consultants. Typically consultants(usually a team) are devs who know how to get things done, including coding, setting up the best practices, adopting agile methodologies. They get the requirements and help move things in the direction to production. They bring in expertise on all areas, not just dev but also product management and everything.
When I worked with consultants, we had a lot of things that needed to be set up and planned for production. We brought in consultants. They helped us, set things up (technology, stack, practice), enabled us in everything and once they left we took on a steady pace.
Pivotal, ThoughtWorks have some of the best consultants. They're engineers who are hired by clients for their expertise, or at least that's what I think. The catch is you'll never know how they will sync well with your team. I've worked with some amazing consultants and some shitty consultants as well.
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