What is multi-user support ?
Multi-user support allows you to create a UNIX-like environment where several users live together in harmony, unable to delete each others files, unable to read those private love-letters of other users. And this even if several users are working on the machine at the same time (on a terminal hooked up to the serial port).
People without a valid login ID and password won't be able to access files you have made private with multi-user. If you make all files private (not readable for others), the only useful thing they could do is boot !