The duplication of DNA as part of the cell cycle is dependent upon melting the very stable DNA double helix, that is separating it into its two component strands. DNA helicases are one of the two types of proteins whose function is to open the DNA double helix in order to expose a template for DNA replication, the other family being the single strand binding proteins (or helix destabilizing proteins). As this protein is required to move forwards with the replication fork, ie. do mechanical work, energy has to be put into the system. This is achieved by the hydrolysis of ATP. DNA helicase can therefore be thought of as a "motor protein".
DNA helicases are also involved in the repair of damaged DNA by peeling away a damaged section of DNA from its complimentary strand. These are typically "bulky lesions" caused by pyrimidine dimerisation (caused by UV damage) or the action of carcinogens such as benzopyrene.
RNA helicases also exist - these are characterized by RNA duplex rather than DNA duplex dependent ATP hydrolysis.