Why does hydration of alkene takes place in presence of an acid such as H3PO4 or H2SO4?
The acid is used as a catalyst. As can be seen from the mechanism below, the electron-rich double bond first attacks the positively-charged acid. The positively-charged hydronium ion is important because it is rather electrophilic and allows the nucleophilic pi bond to attack it, breaking the pi bond. This serves to initiate the reaction. Without an acid catalyst, the neutral water molecule would not be electrophilic enough for the pi bond to want to attack it, thus the reaction would not proceed.  

(from http://slideplayer.com/slide/11423361/)