Many start-up companies are founded by technical leaders - people with deep technical expertise in their field. Once the product(s) are ready to go to market, or perhaps only ready to engage with a beta customer, a new skill set is needed. This may be an opportune time to have an interim Vice President of Marketing.
An interim Vice President of Marketing, often called an "Acting VP of Marketing", can be part-time, so you can access the marketing expertise needed without a large increase in your expenses. You can scale up the amount of time needed and even use the interim VP to help hire and successfully launch the eventual full-time vice president.
Do not make the mistake of assuming that just because you have a good product it will simply sell itself. Most companies only get one chance to do it right. As they say, "You only get one chance to make a first impression." If you stumble on your first approach with a customer you will likely be hampered in all of your later approaches. Instead, you need to carefully build your story and its supporting materials - presentations, datasheets, pricing structure, competitive analysis, cost-benefit analysis, etc. Not every sale will need every tool. But you do need to be properly prepared. That also goes for public relations and the press. You need to get your messaging correct. Without an experienced marketing leader, it will be very difficult to get the quality of marketing output you will need to succeed.
Of course, some companies may need an interim executive simply because the previous vice president left unexpectedly. Or perhaps you simply cannot afford a full-time marketing VP yet. By bringing in an experienced interim executive you keep your business moving forward while you search for the right full-time person to fill the role.