I would think that having a lot of female engineers would help both hiring and retention of female engineers
Probably more important, with retention, are working conditions. If a company has a high turnover rate, women aren't going to stick around just because they're 20% instead of 10%.
Anectodally, women may be less likely to switch jobs if they’re satisfied with their work environment.
...isn't that pretty obvious (and universal)?
Not necessarily. For example I am happy in my current job (it is, without a doubt, the best job I have ever had) but don't get paid anywhere near what I would like to so am actively looking for new work. If rdlecler1's anecdote is true a woman would be more likely to stay and only start looking to leave once the job starts making her unhappy.