Answer 1

They're used case by case, for percision usually.
// returns int and / returns float. Examples:

if i have a program that uses threads, but the amount of values per thread depends on the amount of values given - take the total number of values and hard divide // it by the number of threads!

if i have a class that prints out the percentage values of a job getting done.
Sometimes the percentages can have very small increments depending on the amount of work. Because of these smaller increments, i'd like more percision so i use regular division to get a float value for the percentage.