Why do people write #!/usr/bin/env python on the first line of a Python script?

Question

It seems to me like the files run the same without that line.

Answer 1

The line #!/bin/bash/python3 or #!/bin/bash/python specifies which python compiler to use. You might have multiple python versions installed. For example,
a.py :

#!/bin/bash/python3
print("Hello World")

is a python3 script, and
b.py :

#!/bin/bash/python
print "Hello World"

is a python 2.x script
In order to run this file ./a.py or ./b.py is used, you need to give the files execution privileges before hand, otherwise executing will lead to Permission denied error.
For giving execution permission,

chmod +x a.py