A firewall is an essential element of a security policy for any organisation, small or large.
What type of firewall to run, or whether to choose a hardware or software option, will depend on the size of your organisation and if it has many remote users. For small organisations with maybe less than 10 PCs may find it more practical to have a software firewall on every PC but any more than this will probably necessitate a hardware firewall.
The key to a successful implementation of a firewall is to ensure that it is correctly installed, regularly updated and that you set proper policies to meet the needs of your organisation and users.