I believe the reason they do it is simple…
There is the belief that if one day Britain found itself in such a dire situation that it needed financial help from other countries then these countires who we’ve helped out now would be more inclined to want to give us a helping hand in our time of need. Sadly I don’t see this is a very realistic prospect given the amount of corruption that there is in the countries that we choose to help out.
I did saw an interesting fact a few days ago…did you know that all the cash in the world combined cannot pay off the US national debt?