It's ultimately to keep rules independent and more flexible. Shouldn't use OR's just so you can avoid specifying the same consequence multiple times.
ie:
if:
person.getCreditScore() > 800 || person.getFather().equals( Employees.CEO )
then:
loan.setInterestRate( 0.01 );
Here, the logic is intermixed. There's basically 2 business cases here.
1) You've got great credit.
2) You're the CEO's son.
By using/allowing OR, you might model the situation correctly for Right Now, but what if in 6 months, the loan board decides that the CEO's children have to pay 2% instead, while great credit score folks still sit at 1%?
If you'd avoided using OR, then that'd be an easy change to make. If you used OR, now you've got to break the rule apart into 2 rules anyhow.
Rules should express a single business case, really. Using OR to reduce the number of rules simple because they happen to have the same outcome/consequence will make things more difficult down the line, wrt maintaining the rule-set.
