What can happen to your computer if you never delete anything?
Absolutely nothing. Blitz3D tries to automatically return all memory when the program ends. If by some chance, the memory was not freed, then some memory would remain used and not useable by the system until you restarted or switched off your computer.
It is highly unlikely that you would ever cause any damage to your computer hardware by programming. You can delete files and/or corrupt the registry if you use file or registry commands, but otherwise, no chance. Modern languages, and particularly very high level languages such as Blitz3D, simply don't let you program the hardware so there is nothing you can damage.