An FMC tells you the optimum altitude based on aircraft weight only. Some newer generation FMCs also include forecast winds and will recommend an earlier climb to take advantage of weaker headwinds or stronger tailwinds. They do NOT take into consideration actual conditions unless you tell them what they are. They do NOT take into account traffic issues and going up early to ensure that you can hold your optimum altitude for the remainder of the flight. They do not take into account trading off time/ fuel for schedule and they do not take into account the ride or passenger service requirements.
So I can fly the program as handed down and drop fuel all flight or I can use my experience- something J* management reckons isn't that important- and save hundreds of kgs by getting my altitude early and holding it longer. I can ensure the pax get a smooth ride for the meal service before dropping down out of the winds to ensure an on time arrival or minimising a delay.
Smart pilot managers know that we do all this and don't treat us like idiots when it comes to the savings that actively engaged pilots can bring to the table. Idiotic pilot managers think that the computer tells you all you need to know.