Australia is pretty lax, though they might swab you.

Ugh, don't remind me of Singapore. Transferring after a flight from London, I see a water bottle refill fountain, which I use. We're running late, so I head straight to the gate...and then have to immediately dispose of my newly filled 2L water bottle.

I've never known another airport to have this security setup, and as a massive transit hub, I really can't make sense of why they chose to put it at that point - but then again, what of airport security actually makes sense when subjected to reasonable logic?

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.

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.

In Dubai, which is a massive transit hub, they do the security check upon arrival, before you enter the departures area. They still do some lighter spot checks at the gates, but liquids allowed.

Heathrow as a major transit hub also doesn’t do security checks at the gate, rather it’s when you enter airside. Far better.

Singapore and KL airports are the odd ones out. I suspect because they were built before the security rules required better separation of departing and arriving passengers, and retrofitting the added security paths made less sense financially than just doing checks at every gate.

Kinda like needing to enter your password again to change your account settings if you're already logged in. (I don't necessarily think all these rules are good and just, but if they were, it would make sense to have some redundancy.)

This is not how it works in most of the world. Usually you don't have to go through security when you transit, only when you arrive at the airport. The only time when I had a second security screening during transit was in the US.

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?

Depends maybe on the airport but flying out of San Diego, they pulled a bag of breast milk from my girlfriends bag and tested the liquid inside. They allowed the larger size through but still did scrutinize the actual liquid.