Based on your comment I guess that was my profile :) Personally, the main reason I included that is because I don't care too much which is used, so long as it's not "he/him". People assume I'm male on the internet enough that it kind of annoys me, so I figured I ought to explicitly say somewhere what I am comfortable being called. It's a mix of not feeling strongly about my assigned gender, wanting to be semi-anonymous, attempting to avoid gender stereotypes, and not wanting to argue with people about singular "they".
A similar example is someone I follow on Twitter who used to have a line in their profile saying "Any pronouns are fine". When someone asked about it, they clarified that the intent was so that people wouldn't have to worry about potentially offending them by choosing the wrong pronouns, because they really don't care one way or another how someone refers to their online persona.
That said, I also have a couple trans/nonbinary friends whose first public coming out was to say "hey, it would be cool if people used 'they', but '(old pronoun)' is still ok too". For them it was a way to test the waters socially and kind of try on the new identity, so they certainly appreciated when people remembered and used the new pronoun.
Gender identity is a rather personal thing, so I'm sure there's many other possible reasons! When in doubt, it's usually safe to just ask, so long as you're being respectful about it. I haven't encountered anyone who would be offended by a polite, good faith question like, "Hey, I noticed you list both 'she' and 'they' in your profile, do you have any particular preference?"
If you're not able to ask, it's certainly reasonable to use whichever they've listed as preferences; I haven't seen bad reactions to that. Even one of the people who later came out as trans responded to friends 'correcting' other comments, to remind them that they'd said 'old gendered pronoun' was ok too. However, I do typically use a gender-neutral option if listed, given that multiple choices may indicate they're not entirely comfortable being gendered (and if they're cis, they won't care either way).
One last note: people who use "unusual" pronouns may get questions about it fairly frequently. So even if you can ask, part of being respectful is being respectful of their time and energy used to explain it: try searching first! It's possible this person has already written somewhere publicly about their pronoun preferences, if it's an online profile where you saw this. Or, perhaps another person has written a relevant article or blog, and if that doesn't answer your questions, you can at least show that you've put some effort into learning, and maybe even refine your question to be more specific instead.