JQuery is used due to following advantages:-
Easy to use and learn.
Cross-browser support (IE 6.0+, FF 1.5+, Safari 2.0+, Opera 9.0+)
Easy to use for DOM manipulation and traversal.
Large pool of built in methods.
Methods for changing or applying CSS, creating animations.
Event detection and handling.
Tons of plug-ins for all kind of needs.