Why Apple recommends using Double over Float?

Because it has more space in memory, it can be more precise. Specifically, twice as precise (hence the name "Double")

Computers only have a finite amount of space, so Floats are called Floats because in order to store a decimal number, a computer needs to do what's called "floating-point arithmetic". Basically it rounds the decimal number to the most precise place it can, so 22 / 7 might get rounded to 3.14285707

A Double is basically 2 Floats stored right next to each other in memory, so a Double has twice the memory available as a Float, which means it can be twice as accurate. So, where a Float might be able to say that 22 / 7 is 3.14285707, a Double can say it's 3.1428571428571428

I know a Double technically takes twice as much memory as a Float, but that's not nearly as big of a deal as it sounds. Memory is actually extremely cheap nowadays. The newest iPhones have 4 GB of RAM, which means it has enough space to store 536,870,912 Doubles in memory at the same time before it has to start using swap space on its flash memory, which is orders of magnitude larger. It's likely much more important that your app be mathematically twice as precise than it be space-efficient, so Apple recommends that you always use Doubles