Quick question about soil stack connection

[JayJay1978]

When a 40mm or 32mm waste pipe exists the bathroom and into external soil stack, why is it you see some with a straight slope into strap on boss, and you see some that go down 6 inches or so and horizontal or 6 inches or so and then into boss. I've seen looads like this and it makes you think why use two extra elbows when you could use a single straight pipe.

Is it in case of back up?

Like this.

 

[doitall]

There's a maximum angle on the boss, so if it is too low from where the pipe exits the building you have to step it down, most common with alterations.

 

[JayJay1978]

Ahh, simple as that.
Cheers.