Simple answer to a simple question. With every aircraft order, large artillery order, ship building contract or military tank order, our politicians and officials get a huge kickback. The first notably well publicised scandal was the Bofors arm scandal which involved a famous actor turned politician returned actor. It is not totally about arming India and keeping her on the defensive. It also involves a huge amount of "backhands" which ensures that politicians and officials create a financial dynasty.
Offcourse to the gullible public (as is evident in this forum), it is all about India having one more aircraft carrier than China, a generation or so advanced aircrafts over Pakistan and a few thousand tanks more than Bangladesh. In their lifetime, the chances are that none of the advanced weapons purchased will be used in any warfare against India's neighbors. But offcourse, alleviating poverty doesn't really win you votes or earn you huge kickbacks as buying weapons does.
Hopefully the thread starter will now understand in a nutshell why India invests heavily on unnecessary weapons. It is because it has 1,2 billion or so people to fool