Prototypes can have different purpose, so let's look at the most common cases:
1. The prototype's purpose is to explain the idea
You'll need to be able to explain your idea on several levels.
The elevator pitch does usually not involve showing a prototype.
If you have more time, you should still be able to explain your idea without a prototype, be it with words, gestures or pictures.
If you're uncomfortable selling your idea without a prototype you need to work on how to present it.
In some cases, the idea is so novel that the best way to explain it is to show it. In that case a prototype can be valuable. Before Pokemon Go made people aware of what Agumented Reality might look like, a prototype was probably the best way to explain it.
2. The prototype's purpose is to show that the idea is feasible and that you and your team has the skills to make it happen.
If you are selling to investors, a prototype might convince them that you and your team have the technical knowledge required to deliver. This depends on how complex your idea is though.
3. Customer feedback and iterating
In my experience, the prototype delivers most value when it is used to get real-world feedback from real customers, and when the prototype can be quickly adjusted to accommodate the feedback.
This is very valuable when used to validate product-market fit and to make incremental adjustments.
I have helped IT startups build their first prototype. I've seen that the prototype can be used as a crutch, instead of tackling the real business problems. Make sure you can sell convincingly without a prototype.