It is just easier to write "they" than "he/she"
"I don't know whose luggage this is, but they really should not leave it unattended" is easier than "I don't know whose luggage this is, but he/she should not leave it unattended"
The singular “they” to refer to an unknown person has existed for hundreds of years and is widely accepted. It is best to avoid it in formal writing, just to be safe, because there are some people who don’t approve of it and might judge your writing negatively. But otherwise it’s perfectly fine to use.