It's a little bit like the difference between managing, say, menial labourers versus managing professionals, like software programmers. With the labourers it's very easy to see who's doing well and who's not, and to get an exact measure of productivity. It's also very easy to boot someone out and replace him. In software development it's extremely difficult to accurately (and evenly) measure performance, and you have to be much more careful about how to motivate (and be very aware about the possibility of negatively motivating someone). And a software developer is much more difficult to replace than a menial labourer -- very similar in some ways in how it's easier to replace a minor leaguer than an NHL player.