I will post this
LINK of my attempt to answer this a few days ago.
I competed in the NPC, which was well known for users.
It was very difficult for a non-user to beat someone like myself and I was obsessed with winning. If you asked, "how much did steroids help you?" I would guess only 10% at the most, but in competition 10% can be enough to put a person out of reach for a drug free competitor. Now if the other guy was using it came down to a more level playing field and it came down to who paid their dues in training off season and on season.
So in summary, I suppose you take them because you know your competitors are taking them and you don't train hard just to lose all the time.
I believe most competitors would like to be drug free, but there is always a paranoia if "I show up drug free and the other guy doesn't I am going to lose" so "I won't take a chance that the other guy may not keep his word and I will go ahead and use them as well so that I have a chance of winning."
I'll be honest I hated using drugs and I won almost every competition that I entered, but I couldn't wait to get off steroids after each competition. I used them because I felt like I had to, but I hated every day that I had to prepare another syringe. I bet there are many people like this.