The QS rankings speak for themselves. Apparently the results of the 2018 QS rankings are as follows:
NTU - ranked 11th
NUS - ranked 15th
SMU - ranked 441st-450th

2018 THE rankings
NUS - ranked 22nd
NTU - ranked 52nd
SMU - not ranked

Rankings matter insofar the prospects an average 2:1 graduate from a university has. If you belong to a higher ranked university, e.g. LSE/UCL (and probably NUS given how highly it is ranked recently), a 2:1 from there could potentially reap a lot more. If you come from a lower ranked university, a 2:1 from that particular university could potentially limit your career at the outset. Notice the word “potentially”. Every university (top ranked or bottom ranked) has success stories of sorts.