Why is Session State disabled in SharePoint by default?
Does it not make use of session in any feature of SharePoint at all? If so then which features require session?
With MOSS session state is enabled when you install a Shared Session Provider. Features such as InfoPath Form Server require it. The Session State uses SQL Server as its persistance store.
Out of the box WSSv3 doesn't seem to have any features that require it, though if you need Session State for web parts and you do not have MOSS then this post should help:-
"Service requirements for session state
Both Microsoft Office Project Server 2007 and Microsoft Office Forms Server 2007 maintain session state. If you are deploying these features or products within your server farm, do not disable the ASP.NET State service. Additionally, if you are deploying InfoPath Forms Services, do not disable the View State service."
excerpt from TechNet article Plan security hardening for server roles within a server farm (Office SharePoint Server)