I really don't know what her situation was in previous formulas/racing. it's very possible for someone to shine in a crap car that can't even get in the top 10, it's equally possible she was just crap. It's also pretty likely that even if a woman was doing well enough to deserve a drive in a stronger team that a team might not want to take a chance on a woman. So she may not have had the chances that a male driver would have had.
The general idea is that women may see racing on tv as kids, they are all men, and feel like that sport isn't for them, and that having a female driver who deserves to be there or not, may encourage more women into the sport. For every Alonso there is probably a couple hundred thousand young drivers who turned out to be crap, so for there to be a female on ALonso's level you need to get more drivers to try it so that you eventually get a really good one.
I can see the idea, it's not terrible but then there is no absolute need for there to be a great female driver. Ultimately where the equality/fairness part comes in is less at the top end, but more like not excluding young female drivers by default of it being a male only sport.
If she truly deserves a spot, probably not, but it might be a good step to finding a woman driver who does truly deserve a spot. I don't think it's entirely necessary, but I understand the general idea behind trying to get a bit more exposure of women drivers in the highest level of the sport.