Since you load the user object from the global scope, everything you change on it will affect other functions further down the line.
If you make alterations to the user object that only apply to your module, you might want to reset those changes so other modules won't be able to see the alterations you made. Think about adding something like adding a security key, then run it through some of your functions. You don't want other modules and functions to see the key, so you want to remove it from the user object after your functions are done with it.
For only loading the user and reading it, I'd say it doesn't make sense to reset the variable.
As for your example, to bypass the access check the global user object (current logged in user) is swapped with the object of user 1.
In this case it is very important to switch back, otherwise all other functions will see you as user 1 rather than your own account!
See https://www.drupal.org/node/218104 for documentation on impersonating other users.