If you already have in-house IT services, you may not understand how useful an IT consulting service can be for you. You've already got your well-trained experts, so why would you need to spend the extra money to bring in outsiders? Well, there's a lot of reasons!
In the first place, your team is working on handling everything you've got going right now. They're handling a lot of minor stuff, like recovering passwords or repairing and replacing broken equipment. They're also handling major stuff, like keeping your servers, or your mainframe, operational. They are dealing with a lot of the day-to-day stuff. When you have a lot of people who are working hard in the everyday work of keeping things afloat, they may not have the time or energy to spare to think about how to change things for the better.
Additionally, while your team may be excellent at preserving the systems you already have in place, they may not have the breadth of knowledge to see what new solutions could be implemented.
This is not to say that your IT team doesn't have valuable input on the matter, and your IT consultant will work with them to determine your needs. For example, your team may know that a type of change regarding data storage needs to happen, even if they don't know what the best solution to that problem is. They may even have a list of little, quality-of-life improvements that would make their jobs easier, but haven't had the time or personnel to come up with a plan for implementing those changes.
The consultant may also be able to find some weak spots in your IT department's knowledge base and help your team correct them. For example, if you have recently moving over to cloud storage, you may not have hired anybody who has more than a vague idea of how to work with cloud storage. Undoubtedly they have some understanding of it, but their practical knowledge may be limited to their personal usage. An IT consultant will be able to spot this, and you'll be able to help get your people the training that they need to keep up with your business.
Another thing IT consultants can bring to the board is the ability to strategize. While your IT team may know that changes need to be made, and may even know what needs to be shifted, they are likely not trained in how to draw up a strategy to make those changes fluidly, with as little negative impact to your business as possible. A major technological shift that is handled badly can bring your business to a standstill, as well as frustrate literally everybody who has to deal with it. With a good strategy in place, you can avoid that entirely.