It's fun printing directly from computer, until it decides to go to sleep, decides to install updates, decides to freeze, have communication problems with USB or just slow down for some reason.
That's when your printer throws a fit. It might just reset Arduino or it might stop and continue extruding filament, keep heating untill it melts, etc... Your print is ruined.
It is better to read files/print from SD card, than have a computer sending G-codes layer by layer for hours and just waiting for an accident to happen.
Anyway, you can use Pronterface to print from your computer.
Just disable screen savers, sleeps, updates, reboots... Your computer needs to be ON and working while printer is working.