1) 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, 2588).
2) Why is paying Zakat beneficial for me in the hereafter?
"If you lend to Allah a goodly loan (i.e. spend in Allah's Cause) He will double it for you, and will forgive you. And Allah is Most Ready to appreciate and to reward, Most Forbearing." [Al-Quran 64:17]
"Who is he that will lend to Allah a goodly loan, then (Allah) will increase it manifold to his credit (in repaying), and he will have (besides) a good reward (i.e. Paradise)." [Al-Quran 57:11]
"Verily, those who give Sadaqat (i.e. Zakat and alms, etc.), men and women, and lend to Allah a goodly loan, it shall be increased manifold (to their credit), and theirs shall be an honorable good reward (i.e. Paradise)." [Al-Quran 57:18]
"The believers must (eventually) win through, Those who humble themselves in their prayers; Who avoid vain talk; Who are active in deeds of charity." [Al-Quran 23:1-4]
"Ta Sin! These are the verses of the Quran and the Book that makes (things) clear, A guidance and good news for the believers, Who keep up prayer and pay the poor-rate (Zakat), and of the Hereafter, they are sure". [Al-Quran 27:1-3]
"...keep up prayer and pay the poor-due and offer to Allah a goodly gift, and whatever of good you send on beforehand for yourselves, you will find it with Allah; that is best and greatest in reward; and ask forgiveness of Allah; surely Allah is Forgiving, Merciful." [Al-Quran 73:20]
"Surely they who believe and do good deeds and keep up prayer and pay the poor-rate (Zakat) they shall have their reward from their Lord, and they shall have no fear, nor shall they grieve." [Al-Quran 2:277]
"And keep up prayer and pay the poor-due (Zakat) and obey the Messenger, so that mercy may be shown to you." [Al-Quran 24:56]
"Truly he succeeds that purifies it (the soul), and he fails that corrupts it!" [Al-Quran 91:9-10]
"If you give alms openly, it is well, and if you hide it and give it to the poor, it is better for you; and this will do away with some of your evil deeds; and Allah is aware of what you do." [Al-Quran 2:271]
"But those will prosper who purify themselves." [Al-Quran 87:14]