A typical felt underlayment can absorb moisture that can damage the roof deck. Subsequently, moisture can cause the underlayment to swell, creating a roof that appears warped or wrinkled. Felt underlayments are also prone to other types of damage, such as being torn during installation. All of these problems lead to premature underlayment replacement.
If the roofing underlayment prematurely fails before the roofing material, significant time and money could be spent on a roof that could have lasted longer.