I've often wondered about that in the preflight checks. It seems like quite a dangerous thing to do but there's obviously a good reason for it.
Given you've just taxied out on a fuel tank surely you have reasonable confidence that it's all working (free from blockages, no water in the fuel) and so it's safest to take off on that tank. Then change to the fuller fuel tank when at cruise altitude. That way if there is an issue with the fuller fuel tank you can change back in good time.
The change to fuller tanks seems to me to make it more likely to have an engine failure at one of the most critical times in a flight, during take off
So why is it advised to do it as an essential pre-take off action?