Consultancy sounds fancy, but when it comes down to it it's fairly mundane.
I used to be in consultancy as a software dev, it's basically same job, higher cost. But these companies use consultants because they have tons of money but aren't sexy enough to attract internal employees that do the same thing.
Consultants are easy to fire. Depending on the company's policies, local laws, and the internal politics of the organization, a 'real' employee may be a lot harder to get rid of. Consultants are also time/task limited (in theory), while a 'real' employee is not (again, in theory).
I think this is mixing up 'Management Consultants' vs 'Engineering Consultants/Contractors'.
I'm a (supply chain) consultant which is similar and have a full time contract and full worker protections with my employer. Projects are sold on a fixed price basis, so there isn't anybody to fire - clients are buying the work output or report, not the person or set hours. Some projects are time & materials, but if the project gets cancelled I'm still getting paid - I have a full time contract.
As an example of the value prop - I come with information that your company doesn't necessarily have. Let's say you need to expand your existing (15 year old) miniload ASRS and are locked in with the incumbent provider - I know up-to-date market rates for miniloads and racking, and will be better able to credibly negotiate it down to market rates (particularly as I have probably done a project with the vendor before). I will also be better able to tell you if there is a better solution to expanding the ASRS because, while this is a '1 in 10' year activity for the client, this is what I do every day. Maybe they would be better putting some shuttles in a separate ASRS fed by the miniload because the actual bottleneck is throughput/accessions instead of storage, and their stock profile is different to what it was 15 years ago? They are better than me at running their warehouses, but (hopefully!) I'm better at specifying automation.
So that's the value proposition of consulting - you can get access to very specific expertise that you don't need for long. Most companies don't need a full time expert on specifying warehouse automation!
They usually want their tech outcomes to be more like Netflix, but they don't want to have a tech-culture. So they bring in the consultants to implement "the digital transformation"!
I used to be in consultancy as a software dev, it's basically same job, higher cost. But these companies use consultants because they have tons of money but aren't sexy enough to attract internal employees that do the same thing.