Her accent will be very much to her disadvantage if she wants to be an actress.为什么用to?

to one's disadvantage 是 to one's advantage 的反义词.
前者是”对某人不利“,后者是”对某人有利“.
句子意思：如果她想成为演员,她的口音将会对她很不利.（不利于她发展）
请参考：
It would be to your advantage to agree to his demands.
（同意他的要求,你可能会占据主动.）