Here's why:

Computers are based entirely on mathematical logic.  If you don't use the brackets, the computer will get confused about how to interpret your search request.  Let's look at an example from math:

In the following mathematical question, which do you do first, the multiplication (x) or the addition (+)? 2 X 4 + 3 =

If you said you should do the multiplication first, you are correct.  This is a rule that is built into mathematics.  So in this case, the answer would be 11 (2 X 4 = 8 + 3 = 11). 

 

What if we wrote the same equation differently, however... 

In the following math statement, which operation (x or +) would you do first? 2 X (4 + 3) =

If you said you should do the addition first, you are correct!  Adding the parentheses changes the equation completely.  The parentheses tell you to do the addition first and then to multiply that sum by 2.  So, in this case, the answer would be 14 (4 + 3 = 7 X 2 = 14). 

 

It works the same way with words and search strings.  The computer views your request for information from a mathematical perspective.  However, unlike math, there is no rule about which to do first, AND or OR.  Look at the following search string:

women OR female AND journalists

 

Should the computer look for (1) women OR female first and then combine those results with AND journalists?  Or, should it do (2) female AND journalists first and then look for OR women? 

 

Using the first method, the computer would find articles containing both of the words women and journalists or articles containing both of the words female and journalists.  This is what we want.  We are interested in the topic of women in journalism, so we need to find both concepts - women and female - together with the word journalists.			In the second example, the computer would find articles containing only the word women or articles containing both the words female and journalists.  This is not terribly useful.  Finding female AND journalists is useful, but we do not need articles that are only about women.
women AND journalists	OR	female AND journalists	women	OR	female AND journalists

 

Therefore, the first method is the one the computer should use.  However, it does not know this unless you tell it! 

Our example from above should look like this:

(women OR female) AND journalists

Computers are not intelligent, only very quick at performing the tasks you ask them to do. It is because of this potential confusion that you need to use brackets around  the OR statement in your search string, if you have an AND and an OR together in the same search string.