Can someone please describe why we need Double within the expression?

Answer 1

Double is a cast. The variable is an integer, and will not multiply with a double. To fix this, the integer is cast into a double before multiplying. This causes the multiplication to be done with two doubles.

Answer 2

1.5 is a double,4 isn't. you have to convert 4 to double to multiply. hope I helped :)

Answer 3

You use double, if the expression includes a string literal.
Source:https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/StringsAndCharacters.html

Answer 4

I Believe Double is basically used to return a double value.

Answer 5

I believe that it allows swift to read and process decimal numbers. With out using "Double" swift only reads whole numbers.