Evening primrose oil is good for lots of things.
Stronger bag of waters - I experienced this personally as I had my bag break before labor (past term tho) with both my sons. With DD I took EPO for several months prior to her birth and had a very strong bag. It's not really a big deal, but saves worrying about PROM.
Softens cervix- that's the big thing for a lot of people. It get your cervix softened pre-labor so there's the potential for prelabor dilation or to get things going later on. I also found it worked for me: with both boys, my cervix was maybe halfway effaced before labor. Not dilated at all. With dd, it was almost totally effaced and I dilated to a 2. Not a lot
but better than nothing
because I really labored thru those first few cm with the boys. With her it was a free ride!
And there's also mood enhancing benefits
I took 1000 mg capsules, just 2 per day thru months 6 and 7, then 3 thru the last month. Once I was term I was taking 2 caps 2x per day. Some people insert them vaginally as welll; I tried it once but just forgot about it most of the time.