Because of the following scenario:
Malicious user is able to put malware into your home directory, with the same filename as a common command - "ls" or "cd", for instance.
If just typing the filename worked, then you could run the malware instead of the proper file - infecting your computer.
By requiring the ./ at the front, which specifies the current directory, the above scenario can never happen, as you would have to type "./ls" instead of just "ls"
Good luck with the homework