Using the right amount of fuel is not necessarily about cost savings. The more fuel you load, the heavier the plane is and the more fuel it requires for take off (it's exponential). Secondly, in case of an issue on take off (or shortly thereafter) that requires the plan to land, the plane needs dump some fuel in order to land safely (otherwise it would be too heavy and the landing itself would be more dangerous). The key is to find the sweet spot: enough fuel to make it to the destination, some extra fuel in case the plane needs to be in the air longer (either to circle around or to divert), yet not too much.

I've been in a similar situation on a KL flight LIS-AMS. There were storms around AMS and traffic was restricted (a bit of an air traffic jam). Our flight would not have been able to land right away; it would have needed to circle around for a while, and that would have exhausted the fuel (or come dangerously close). We landed at RTM, refueled and continued the flight to AMS. I'm guessing that something similar happened in your case. I don't think the crew normally chooses to divert unless they feel that it is necessary.