By adding an iOS icon, anyone who decides to save your webpage to the home screen of their iPhone, iPad, or Android will see a nice app-like icon rather than an image of your page.
Here is a table of the current iOS icon sizes from Apple:
|iPhone 6/7, iPhone SE||iPad / iPad Mini||iPad Pro||iPhone 6/7 Plus|
For Android icons you’ll want to follow these specifications:
|Android Regula||Android Hi-Res|
Make the first apple one the 120 x 120.
If you don’t want to create all these images, you should at least create the larger resolution ones. That way they’ll look good on the hi-res devices.