I think sometimes in the case of "Set down only" allows the bus to not run or take a different route if there is no-one on board that requires the full route.

Such an example would be the FromeBus service 960 17:10 from Trowbridge to Westwood, after the Town Hall stop is destined to work the first part of the Town 60 Route to Studley Green, however if no passenger is on board or requires those stops, the bus goes a more direct route.

Set down only could be utilised to ensure that people shouldn't really be expecting that particular bus to either appear or pick them up.

 
And Skylark Road Melksham ... from Bus Times



Which, however, does not tell you that the 14 only goes that way if there are people on the bus who want to get off there or at Buzzard Close.  I guess it probably would pick you up if you risked waiting for it there - the regular driver and most of the stand ins are happy blokes and understand customer service.  But I have never risked it.

 
Ah ... Manvers Street Bath.  Faresaver 271/2/3 will drop you off there right opposite the railway station and it's magic for the transfer from bus to train.  But they don't / won't pick you up at the stop across the road - though most other buses pick up there.

So - arrive from Bristol on the xx:00 train into Bath Spa station at xx:12 and dash across the the bus stop to have the bus sail past you at xx:17.  Frustrating.   Official way is to walk to the bus station on leaving the railway station.  If the pedestrian crossing just before the bus station is at red, chances are it's to let the the Melksham bus leave and turn right towards the railway station.  If you can get across the pedestrian crossing at green and pick your way into the bus station, you'll probably get to the 271/2/3 bay as the bus reverses out.

 
Something that would set people aback if they do not frequently use buses outside London - stops that are set down / pick up only.

(There are bound to be some in London too for that matter).

Thinking of Bath's London Road, there are a sprinkling of buses that are set down only on the way in to town, pick up only on the way out, and even bus drivers have problems with this - waiting at one of the afflicted stops this morning, by chance there was a massive hole in the buses serving the stop, and in the ~20 minutes I waited (just long enough to walk to the bus station, but with buses scheduled to call within 5 and 10 minutes I thought it would be ok).

So... while at the stop, no fewer than three set down only buses went past. The 6, on a diversion as Milsom Street's closed for the Christmas market... sailed past... sort of fair enough - though it did stop to let people off at the following stop by the Hilton. The X76, in from Marlborough on its daily run to the RUH, marked to set down only - though I've previously found that it also is reluctant to stop at its 'Pick up only' stop on the far side of the road. This has previously been discussed on the forum, which put the money on an anti-competition agreement between Swindon, Faresaver and First.

Then, a bus service from Libra Travel that I'd not seen before, but it turns out that it runs Wednesdays only to the RUH and is set down only for the London Road / Walcot Street stops. 'A bus' I thought, and hailed it, and the driver had almost come to a halt before he recalled the 'Set down only' part of his morning and floored the accelerator.

Finally, a bus came in to sight that had the stop on its schedule and thank goodness for cheerful Faresaver drivers.

Mark

P.S. The best way to hail a bus in Bath is to actually park and block the road. In which case of course, you have a vehicle and will not need the bus, but you can certainly gum up the roads, traffic in town was dreadful and the cause was the roads equivalent of plaque in arteries. Also, something odd had happened to a single decker at one of the stops nearest the station - it had ploughed into the raised pavement - hope the driver was ok - a commercial breakdown wagon was extracting it a good few hours later. (There was a time when the Bath bus garage would have had its own kit attend for that, but perhaps times have changed.)