Obviously it wasn't for "no apparent reason" unless you have zero understanding of the conflict.
If the IRA's goal was to kill civilians, why would they phone in warnings? Clearly in the great majority of cases, their attacks were aimed at economic targets.
Whether you agree with it or disagree, the bombings on the mainland were clearly part of a campaign to pressure the British to withdraw from NI. An argument can be made that the IRA would have been better off only targeting the mainland and not their own backyard. The canary wharf bomb would suggest it would have been a better strategy.