Why do you need an architect?
You wouldn't dream of building a house without employing an architect. Yet many organisations make a much bigger investment without using a specialist. So what are the reasons why they wouldn't and why might they have got it wrong?

"That's going to be taken care of by my supplier"
Suppose that you've contracted a third party company to come in and deliver a new system. But are they really going to be looking at your bigger picture? You can be sure that their priority is on their system, their chosen technology, their deliverables and their invoices.
Long before you select your vendor, you need to assess whether what they're offering fits within the long-term strategy of your organisation. And once the selection is made, you need to ensure that their new system is going to fit in with your existing landscape.
Otherwise you spend time and money and effort trying to fit a square peg in a round hole. And you still won't succeed!

"I have someone internally to take care of that"
Well we all start somewhere and I'm certainly not going to cast aspersions on anyone else's fitness for the role. If your organisation is big enough to warrant a full-time architect, then you don't need an external consultant. But if not, I would ask: have you made the investment in time and training for that person to take on what you're asking them to do?
What I can offer is to work with and mentor individuals and teams during the lifecycle of a project. This way I can offer them a head start, some tricks of the trade and some suggestions for the way forward. For their employer I can help assess suitability for the role and work together on a roadmap for their future development. All while offering the safety net of experience to ensure a successful delivery of your project.

"There's not really much IT involvement in this project"
Any substantial change of business processes or introduction of new business models requires a through A to Z analysis of impact long before a final decision is taken. Business architecture is not about systems and technology, but about taking an approach to your business as an integrated whole. You can't expect managers and executives to be up to date about all the new possibilities that developing technology offers, no more than you would expect your techies to be thinking of new business lines and market positioning. A business architect is there to guide you through the jungle of possibilities and find the path appropriate to your organisation, your strategy and your budget.
So I can offer a "sanity check" approach to your planning. Before firing the starting gun on a  big project, take a moment to double check the thinking. And if it is the case that you really don't need an architect for the project, I'll guarantee you that I won't be trying to sell you something you don't need. But the least you can be  sure of is that you'll get a few interesting thoughts and suggestions that should enhance your chances of success.