I personally think that most people put screws in when they are building the swan neck to stop it all coming apart when it is hanging from the outlet before putting the downpipe on.
We have literally fitted thousands.
Basically they are all designed to fit together with no sealant or screws .
It’s about cutting it all to the right length so it’s all tight fitting.
You’d be surprised how many people don’t actually know what they are doing properly.

When properly fitted. There is no need for sealant or screws to any upvc joints in gutters or down pipes. Upvc joints are designed to allow for expansion and contraction with changing temperatures. And any mechanical fixing would prevent this. And thus put undue stress on the upvc by not allowing its natural movement to occur.
This movement can often be heard. It's the cracking noise you often hear from your guttering on a warm day.

Swan necks or any part of the down pipe should not really be screwed together as this will hold any debris that comes off your roof for instance leafs and moss can get caught on the inside of the down pipe where the screw has gone thru.
Silicone should not be used either as you might want to take them off in the future to clean them, water travels with gravity and down pipe connections are designed to make the water flow in the right direction if fitted correctly.

You can not screw a swan neck for derbra getting clogged up in it

I find the best way is to use a silicon sealant it just to stops everything from moving and leaking.

Its a common practice & if screwed in the correct place it should not cause any water leakage.
Good luck Alex

I disagree with some of the answers given on here. A short screw ( preventing any debris on the inside getting lodged ) preferably stainless steel can be fitted through a swan neck bend causing no problems at all . This will stop the swan neck from coming adrift which can happen ! To back this up ,'Terrain' swan necks have a hole factory made for this purpose.