We never start an app project without at least wireframes and ideally a full prototype. There are several reasons for this:

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The user experience is the most important factor in an app's success so we must start by being laser-focused on this. We shouldn't allow technology (or its limitations) to come before the user journey.

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It is easy to change wireframes; to re-order them, move buttons around, add comments to them. Rather do this before any code has been written.

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There is no-where to hide- it really helps define the scope. A protoype should have all the core screens showing all the main functions. If it's not there, it's not in scope. If it is there, we need to work out what it must do and how long that will take to build.

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It gets everyone on the same page. You, your stakeholders, our project managers, designers and developers all know what the deliverable is.

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If you need to get investment, stakeholder approval or just something to show to partners prior to the launch, it is a great sales tool that can be installed on their phone with just a couple of taps and they will understand the purpose and value of the app (and your competence to deliver it).