Why do some traps have an inner spigot?

[WaterTight]

Elementary question I'm sure.. I've ended up with a huge bag of random bits and can't remember if they were sold and packaged as for some distinct purpose...
Shown here posing next to a spigotless trap.

 

[kay-jay]

think its there to stop the pipe being pulled out of line but the others are fine as long as you get your lengths and angles right

 

[stevetheplumber]

i prefer the spigoted ones always seem stronger

 

[kirkgas]

the one with the spigot is a McAlpine trap, by far the best on the market, they will stay leak free for 20 years or more, the cheaper versions wont

 

[dontknowitall]

Because they're bisexual?

 

[mikegas]

its to give a better hold on push fit pipe

 

[turnpin]

I think it is to make a more stable central connection of the pipe into the trap some terrain traps used to supply an insert into the pipe before fitting it into the trap to take up any uneven cut ends regards turnpin

 

[Dannypipe]

What Kirk said.

 

[SimonJohns]

I've always thought it was there to aid sealing and alignment