purpose of shebang line

Lets say I write a python script, test.py,

#!/usr/bin/env python
print('text')

And I run it in my terminal by typing,

python test.py

The test file works with or without the shebang line.

From what I understand, the purpose of the shebang line is to tell the shell which interpreter to use (if running unix/linux).

So if I type into my terminal,

test.py

Then the file should work, right? On my Linux box, this doesn't work. In fact, the #!/usr/ gets replaced by +q6b75.

Is my understanding correct? Am I running the script correctly? What is the purpose of the shebang line? When does one need it?