When should you use the function "IF"?

Whenever you want to present some text according to a criteria: if the criteria is met then present this text, if the criteria isn’t met present that text.
Or if you want to calculate something, depending on a criteria: if the criteria is met then calculate this, else (the criteria isn’t met) calculate that.

Examples: Using IF with texts:
If a vehicle price is higher than $40,000 then write “It is too expensive”, else write “It is affordable”.
If the number of students in the class is greater than 30, write “It's over the limit”, else write “It's still within the limits”.

Examples: Using IF with calculations:
If a worker’s shift was longer than 10 hours, his daily payment should be calculated by consideration of extra hours, else it will be calculated as regular hours.
If the deposit in the bank is greater than $100,000, then it deserves an interest of 4%, else it gets only an interest of 3%.