One of the most important parts of your bike a clean chain is an efficient chain. A smooth & clean drivetrain will make your bike more efficient & maximise your effort.
Your bicycle chain goes through a hard life, having to cope with the power going through it, moving across the cassette, and getting covered in all sorts of road grime. That is why it is important to look after your chain, ensuring it is clean and well lubed. If this is not done, they can wear themselves out, as well as other costly items like chain rings and cassettes.
It is best practice to replace the chain regularly, as otherwise cassettes and chain rings can also wear out. Regularly changing your chain will keep your maintenance costs down.