[Technologist]

If I remember correctly, they acted as a l signal for some boxes to switch aspect ratios between 16:9 and 4:3.

On Sky SD transmissions the STB does its aspect ratio switching signalled
By Wide Screen Signalling in the first half of line 23
(The second half is active picture)
DTT and Freesat use the Active Format Description in the MPEG stream so you cannot see it .

Some tv set blank the whole of line 23 from being displayed
Most blank all the earlier lines so you did not see teletext etc
which is on lines 6/319 to 22/335.

If you want to see the specifications
WSS www.etsi.org 
AFD in annex B ..this is a very long document !
www.etsi.org 
Allocation of services in the VBI ..this is very readable
www.etsi.org