To put it simply, flood vents are like old-fashioned crawl space vents, but they serve a different purpose and are often designed to be closed until they are needed these days. With that said, there are times when crawl space vents can be converted into flood vents.
Ventilation
In the 1950s, ‘60s, and ‘70s, it was very common for properties that had crawl spaces to have vents installed as a matter of practice. These vents were open to the air at all times and were designed to increase internal airflow. This was thought to prevent issues with dampness and humidity in the property. These vents would allow water to pass through them in the event of flooding, which is why they are partially effective in the event of a storm surge.
Flood vents, by contrast, are not designed to ventilate a crawl space. Their only purpose is to give way to water when there is a serious flood. We now know that exterior crawl space ventilation is more likely to cause damage to a property, and can be the root cause of issues with dampness, humidity, and condensation in a home.
Pressure Relief and Equalization
Flood vents are also precisely designed and placed to give the best possible effect. This may seem like a very small difference, but it is important. When a storm surge roars through any area, the margin between endurance and catastrophe can be incredibly slim. This is why FEMA has very specific requirements for the size and placement of flood vents; they know what works best.
In the end, the purpose of a flood vent is to equalize the pressure inside and outside of your property to prevent the flooding and storm surge from causing your property to shift, crack, or even experience localized collapse. This is why flood vents come in many different forms. They need to be functional for whichever home they are being installed in.