First: To add new transactions in blockchain. The block chain serves to confirm transactions to the rest of the network as having taken place. Bitcoin nodes use the block chain to distinguish legitimate Bitcoin transactions from attempts to re-spend coins that have already been spent elsewhere.
Read more here: https://en.bitcoin.it/wiki/Mining

Second: Bitcoin uses the hashcash Proof_of_work function as the mining core. All bitcoin miners whether CPU, GPU, FPGA or ASICs are expending their effort creating hashcash proofs-of-work which act as a vote in the blockchain evolution and validate the blockchain transaction log.
Read more here : https://en.bitcoin.it/wiki/Hashcash