Bitcoin is created by the act of mining. Members of the network provide computational power to solve mathematical problems, and for this effort they are rewarded with bitcoins. The more members are mining, the more competitive it gets. Computers are very good at solving these problems, so the Bitcoin network deliberately makes them harder over time — otherwise all bitcoins would be mined in a couple of minutes. That is why there is specialised hardware on the market whose only purpose is Bitcoin mining.
New models come out frequently, so miners must regularly upgrade their gear to stay competitive. Once a new mining rig is introduced, older ones quickly become outdated and unprofitable, and the gradual improvement of technology increases overall mining difficulty again. The result is a never-ending need to optimise — which is why it pays to start with the best available hardware.