Its power dynamics. If she feels like she doesn't have to fight for you, she'll easily leave you. You're less valuable. However if you're also out with other girls, that you also have options, she's more likely to stick with you. The idea of allowing your partner to flirt with others in a LDR is to let them get rid of frustration in a controlled manner. It doesn't always work, but at least when you lose her, you'll also have been out with other women, so you don't have to be so damn depressed.