remove if function_exists wrappers

I’m just a beginner in theme customizing. I uploaded my theme the other day, and got requests as below:

header.php: Please remove all if function_exists wrappers from all sidebar calls
footer.php: Please remove all of the if function_exists wrappers from the sidebars and use if dynamic_sidebar instead.

I don’t quite understand this, could anyone can help explain them more in detail?
Thank you very much!