Cyclists tend to ride up the inside of vehicles queuing at traffic lights. That can result in lots of cyclists, one behind the other, at the lights. If they're all going straight on, but the lead cars all want to turn left, the result is either a possible accident between a car and a cyclist, or the halting of all motorised traffic until the long queue of cyclists gets clear of the junction.
If the cyclists are bunched together across the front of the queue, left-turning vehicles will have a clear path much sooner (and with far greater safety). Cars going straight on will probably only suffer a minor delay.
Chris