Firewalls come in many different flavors and can be configured to perform many different functions. You can choose to permit only email through them, which protects the network against any attacks other than those against the email server. Others provide less strict protections and block only services that are known to be problems. Generally, firewalls are used to prevent unauthenticated interactive logins from the outside world. This, more than anything, helps prevent vandals from logging into machines in your network.
More elaborate systems block nearly all traffic from the outside to the inside.
However you choose to configure your firewall, the best practice you can have is to install one and make sure it does all it can to protect your business.