Why and when we need null theme?

Hi
I want to know why and when we need null theme to use on our website? What will happen if the theme is not a null theme ? Can anyone pls help me understand this? Thank you very much

nulled themes are premium paid ones which you need to pay to obtain and use

One of the main reasons (there can be many other important reasons) to null a theme is:

Stop 'Call-back Home' function:
There usually is/are some script/s inside the theme that will enable the theme creator to find the site that's using the theme. If you are using a pirated theme, you obviously need to at least remove/NULL that/those script/s. Reasons to do that action is obvious.