Back in the day when women and girls wore dresses, women's bikes were created to have a lower bar so their skirt or dress could easily fit over the bicycle. Nowadays, women's bikes aren't necessary, unless you are planning on biking in a ball gown. If this is the case, get a women's bike! Women's bikes also often have a more upright sitting position, whereas men's bikes are often more hunched.
I am a woman and I have a men's bike which works well for me.
I hope this helps!