Pakistan became a declared nuclear state on 28th May 1998. The main reason why Pakistan has to acquire nuclear weapon is to defend itself against it's arch rival India who became a nuclear power in 1974 and conducted nuclear tests again on 11th and 13th May 1998. The second nuclear test conducted by India in May 1998 left Pakistan with no other option but to conduct the test in response. 

As a neutral analyst, I would say that acquiring nuclear weapon by Pakistan has helped in balancing the power in the region and since then both countries never crosses the red line because they know the consequences.