There's also practical benefits. As someone without children I'd really like to see more funds and attention paid to education because I'm getting really fucking sick of interacting with stupid people.
Well I'd rather vote third party than leave it blank. Blank shows you don't care while third party says you kind of care just ignorant enough to ignore voting 3rd party only benefits Trump
And even if you don't have a few of the immutable things here such as the colour of your skin..
These situations could always happen in the future. Your brother can find out he's gay, your parents might get sick with cancer, you might fall in love with an immigrant, climate change will have an effect on you, and so on
Not only that, but any of these could be you too. You might get mental health issues or get physically sick, you might find out something about yourself, even well into adulthood, or lose your job, or face a natural disaster, or any one billion of things that you need support for
Humans are not evolved to be truly independent. We all need, or will need, support.
This is the veil of ignorance you can use when considering what a just society should be like when making decisions like voting without knowing where in that society you would be. It's a tool to help with empathy like the opposite of fear and rage propaganda trying to make you make stupid decisions.