Why is paying zakat beneficial for me in this world?

Allah (The Glorified and Exalted) says in the Quran: "You will not attain piety (righteousness) until you spend of that which you love. And whatsoever you spend, Allah is aware of it." [Al-Quran -3:92]

"And whatever you lay out as usury, so that it may increase in the property of men, it shall not increase with Allah; and whatever you give in charity, desiring Allah's pleasure-- it is these (persons) that shall get manifold." [Al-Quran 30:39]

"Allah will deprive usury of all blessing, but will give increase for deeds of charity: For He loves not creatures ungrateful and wicked." [Al-Quran 2:276]

Additionally, it must be remembered that the Prophet (peace be upon him) said: "Wealth never decreases because of charity." (Muslim, No. 2588).