The question should be, what benefit does a determined sex have. Being a hermaphrodite doubles you're chances in a partner.

My guess is, the more complex the organism the harder a sex change is, so it's only beneficial in certain circumstances. Snails for example move very slowly and their senses aren't the best, so it's good if you can make children with every other individual of your species that you meet.

Sex change can often be found in fish, ocean is vast and it can be dangerous to venture out of your part of the reef, so it's great if one of your buddies can become a female when your original female dies. Or you're a male moray eel and never get to make little morays because your the runt of the reef, your genes would be lost, but at the end of your life you become a female and bam youre genes get to live on.

Then you have species with no need for sex, prospering with only female members.

Also male and female isn't answered with a simple XX or XY chromosome pair. There are species of frogs where in the north XX are female and XY are male, but in the south XX are male and XY are female and in the middle 50-50 chance. Sex isn't determined by only one chromosome. It's far more complicated and can differ between species.

Most of the time the answer to such more general questions is because it works and isn't a deadly disadvantage. To get a more specific answer you'll have to look at a more specific scenario (aka a specific species or environment).