Zakat that has roots in the Arabic word zaka which means to purify. So when we give charity we purify our wealth by giving what we are greedy withhold.
When you pay zakat you are giving to the poor help support those in need and this helps re-distribute the wealth to the poor.
Also we will be rewarded for the charity we have given - one good action in Islam is rewarded ten times more than previous nations of believers. So one pound of a Muslim is worth more than 10 pounds of previous nations.
So even though you are giving here - you can going to get back over there - on the day of judgement. That's the generosity of God/Allah because you give now and later he will give us much more than what we have offered. Purely because of his generosity.