Your bench players are not only there in case someone gets hurt or for your starters bye week. Another important part of your bench is matchups. What I mean by matchups is say for example you have a good quarterback, not amazing but just good. You also have a slightly above average quarterback on your bench. It is important to look at the matchups for both quarterbacks, meaning look at the teams they are playing against. Do some research about their defense and whether or not they give up lots of passing yards or not. If your good quarterback is playing the best defense in the league, you might want to consider playing your backup. This is why the late rounds can be so important of a draft. You need to get quality backups who can be a starter a couple of times a season.
You will make mistakes sometimes, trust me it happens. You play your backup and your starter ends up doing great. My rule of thumb is if you are unsure who to play, stick with your starter. Basically the same philosophy that NFL coaches use.