While your perdiem isn't income (as skyhigh said) if you were living at home eating bolagna sandwiches you'd still need to buy the meat and bread, so, if you live out of your food case on your trips, it is extra money going towards something you'd need anyway. Or in my case, (as someone who does enjoy eating out) It's not bad to know that you're eating for free, i can eat those bolagna sandwiches when i get back from my 4 day. So, yeh i'm a perdiem man....

Something interesting though, at 25000 a year, you'll notice your perdiem more than you will once you make captain, as an FO i lived off the stuff making my paychecks managable. As a CA, i typically don't even notice that i have the extra in there.