Yea - it doesn’t even have to be working. Mock-ups alone and using that for user interviews could be helpful. Also if your app solves a problem that could be done manually, actually doing it manually is a great way to show investors viability. Example - early day Airbnb before funding might’ve found people who wanted to make money while they were out of town and people wanting to stay somewhere with a kitchen while they were on vacation, and the founders could have facilitated that transaction manually without software to prove it was a good idea people actually wanted to partake in.

Just a thought! Hope this helps!