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The purpose of underlayment in roofing primarily serves to provide water resistance. Underlayment acts as an additional layer of protection beneath the roofing material, preventing water from penetrating the roof deck in the event of leaks or heavy rain. It is typically made from materials that are impervious to water and are designed to channel any moisture that might reach them away from the underlying structure, thereby contributing to the overall durability and effectiveness of the roofing system.

While underlayment does contribute indirectly to other aspects of roofing, such as supporting shingles and providing some level of temperature control, its main function is to act as a barrier against water infiltration. Its importance cannot be understated, as proper installation of underlayment can extend the lifespan of a roof and help prevent damage to the structure below. In terms of aesthetic appeal, it is generally not visible once roofing is installed, so this would not be a primary function of underlayment.