The term "100-year flood" is often incorrectly used and can be misleading. It does not mean that only one flood of that size will occur every 100 years. The term is a statement of probability that scientists and engineers use to describe how one flood compares to others that are likely to occur. Today, we use the phrase "1% annual chance flood." What it means is that there is a 1% chance of a flood of that size happening in any year. Over a 100-year period, it has a 63.5% chance of occurring. Even more surprising is that over a 30-year period (typical mortgage period) the 1% annual chance flood has a 26% chance of occurring. This means a home in the mapped flood hazard area is five times more likely to be damaged by flood than to have a major fire!
To answer your question about why you need flood insurance, you would need to look very carefully at what caused the flood and how high the water near your home rose. Because rainfall amounts are different when a storm moves across an area, a "100-year flood" may occur in some places but not others. There are many factors that can add to flooding, including debris in the waterway, small road culverts and bridges, frozen or saturated ground, and others.
If your area had a major storm and your home was not flooded, you may want to check with your community's engineering or planning office. If other areas didn't flood, it may mean that the FIRMs should be revised. You may also want to check to see if your home is eligible for a Letter of Map Amendment (LOMA) which FEMA uses when homeowners submit Elevation Certificates showing that their homes are out of the mapped floodplain. With a LOMA, your lender may choose to not require flood insurance.