Just as Microsoft's products have become the de facto standard with daily desktop tools such as Word, Excel, PowerPoint, and Outlook, SharePoint is becoming the de facto standard web platform for team and company collaboration. There are other products that provide collaboration, but few integrate as seamlessly with a company's existing IT investments just as Office, Active Directory, Windows 7/Vista/XP, or SQL Server has, thus making the deployment process rather palatable to the IT department and workers within a company.
Another huge benefit of SharePoint is that it seamlessly integrates with the desktop technology of Outlook, Word, Excel, Access, and Visio, so an end user's acceptance is much easier and quicker as they feel that this is an extension to their tool set, and not a replacement.
Our rule of thumb is that if you are receiving more than 100 e-mails a day, you probably need a better tool than Outlook to be more organized.