Submarines for defense or offense?

The purchase of submarines seemed to have been a done deal, but then was halted due to public outcry.
In my view submarines are mostly an aggressive weapons, with limited defensive use.

The timing for such a purchase is inappropriate, with the Thai economy stalled and probably the most pressing issue should be drought - flooding - reservoirs - water supply 

I suppose some consideration should be given to "The best defense is a good offense"

The Bangkok Post article presents a somewhat official line of thinking with thought provoking reader comments

Navy claims lack of subs puts it at regional disadvantage

The Royal Thai Navy has published its position paper on why it sees an urgent need for submarines, arguing they’re necessary to maintain parity with neighbouring fleets.
BANGKOKPOST.COM|BY POST PUBLISHING PCL.