Lots of reasons. For example, you can't just paste a picture from your Clipboard into a IRC channel and expect it to work. Granted there are image hosting services out there, but if you want a chat server on your private network and you are a small company, you want it all in one. Same goes with highlighting mentions, Desktop notifications, saved transcriptions (IRC is transient by nature), LDAP/Kerberose integration, XMPP support (again, there are XMPP bridges for IRC, but not integrated in any free private IRC server I know of). UI is another aspect too.