Imagine a security guard at an office building. 90% of the time might be sitting in a booth watching monitors. The other 10% of the time is buzzing people in or making rounds. But that 90% of the time when they are sitting around is important, since they are on call. So you are like a security guard. Your employer gladly pays you for the 90% of the time you are doing nothing, because they need the comfort of having a developer on call for when something needs to be done ASAP. They won't have to go to Upwork or wherever to find someone because you're already there. As long as you are 10x 10% of the time, you average out to a competent FTE and you are tolerated.