It is a plug'n'play email and cloud storage software appliance that installs in minutes. Requires zero maintenance and you have email data encrypted at rest and in transit. You own it and control it.
Alternatively a user will have to use various open source or commercial software packages (i.e. operating system, mail server, anti-spam, anti-virus, cloud storage, archiving, encryption) and try to make them work together. It is obvious that an alternative approach needs highly skilled technicians, lots of time, requires high maintenance (time and money) and the results are uncertain.
Not only this, a custom made solution will still be missing some features found only on Starkit such as the unique ultrafast webmail interface designed for productivity, remote appliance unlocking, unified user management and automatic upgrades.