I think the approach was to disperse the security screening to gates so that you don't have to process them all before they even enter the departure area.

But to your point, it’s terrible. They basically screen you, then you sit in glass enclosed area, with no toilets and you cant bring liquids in, but there are water fountains.

So if you have to use the bathroom you have to leave, then go through security again. Or if there is a gate change, you have to go through security again.

It creates more problems than it solves.

Terminal 4 does it the normal way with screening before you reach the departure area. They also have scanners where you can leave your laptop in your bag.

Much better.

One benefit of distributed checks is that they are more predictable: doesn't matter how busy or not the rest of the airport is, your queue length doesn't change. TXL had that setup (independent security for a pair of gates each) for the older areas and leaving from those terminals was really fast and reliable. Sure, duty-free was tiny then (since also only for a pair of gates), but that never really mattered to me.

I guess for a hub like Singapore it might also make it easier to adjust checks done for different destinations?

I think the reason is, like you said, it is a transit hub.

So passenger flying out of Singapore can mix with people in transit. Therefore, putting the scanner before boarding prevent a possible terrorist attack when someone smuggle in explosive or what not from Singapore or from transit flight.