It is a 15A fuse, which means it will run 15A without blowing. That is close to 200W on a car. 15A is the current at which for all practical reasons it will never blow (although never is a really long time). You can easily pass 3x the current and the fuse will still not blow, but after a few days or weeks it will. The reason for different rating fuses is because a short circuit still have some resistance. Making a short circuit with a rusted screw driver will draw less power than when using a bright new spanner. Also the wire thickness on a circuit might mean that a perfect short at the end of the cable still limits the current to only 20A or so, in that case you would use a 1A fuse. There is nothing wrong in using only 1 fuse for all the brake lights, however it would be nice when the ECU manufacturer (which is not Volvo) would include testing the brakes circuit.

Basically a fuse only protects against a short circuit, so as suggested by someone else there might have been a short on the connector when the OBD2 plug was used. It is unlikely anything else caused the fuse to fail.