It's all about:
1) The construction itself, a project for various cronies.
2) Raising the price of the land wherever the MRT line goes, benefits the real estate industry including the HDB.
3) Increasing the supply of a public good (public transport) for an anticipated higher future demand (increase in population)
Reliability is irrelevant, the elites do not take public transport anyway, unless it is for some special cringe photo op session for the media cameras.