If the clutch is used up (not prematurely) the car and thus the gearbox probably has a lot of miles on it. If you then choose to change the clutch, a small investment, it'd be dumb not to also change the gearbox oil right away. The oil quality does degrade over miles, and to preserve your gearbox and smooth shifting, it's wise to change the oil. It's mostly less than 2 litres, and hence inexpensive to do.
At the same time, while draining the oil, the mechanic will observe the magnetic plug and the oil coming out of the gearbox. Sometimes a lot of metal debris is present in the oil and on the plug, and then you know the gearbox isn't going to live much longer. That's good to know, and may likely greatly influence your investments in reparations.
All in all, a small but wise investment in your car.
It's common practice in most garages.