Because "plz gimme teh c0des for a plus b divided by c minus d" is not likely to be a-d+(b/c) . Just like 1 divided by 1 plus x != (1/1) + x . And it is not the absolute truth, but supposition.
Parentheses are only needed if you need to override the normal order of operations. Multiplication and division have higher priority than addition and subtraction, so
P = w + x/y - z
is the same as
P = w + (x/y) - z
which is the same as
P = (w + (x/y)) - z
Basically, without any parenthesis, the program will first divide x by y, then it will add that to w and lastly it will subtract z.
If you need it to do things differently then you'll need parenthesis. But you didn't really provide enough information to know what order you need the things to be carried out in.