It is simply a value judgement by the members of the council. The LTN on this includes the following warning to those making the decision:-
Expenditure incurred by a council under s.137 is open to challenge by the auditor, or by a local government elector objector at audit (s.27 of the Local Audit and Accountability Act 2014), on the basis that the expenditure is larger than the direct benefit to the area or to residents would justify. Councils must therefore exercise care when considering the amounts of proposed expenditure under s.137 and, if in doubt, should seek advice before going ahead.
Not sure from whom we're supposed to seek the advice, but hey ho!