In the OS 1:25000 map below I have circled some numbers next to roads. The key says "Ground survey height" and there is another entry (in brown) for "Air survey height", but since all the data in the map comes from a survey of one kind or another, I'm not really sure why this is relevant. You can already tell the height within 5m from the contours, and you can make a reasonable estimate if you want more precision than 5m.
I have tried to think what they might used be for, perhaps for knowing drainage directions, but it seems too little information for that (i.e. you'd need a more detailed and specialized map for that). So...
What are these specific heights used for (why do they appear on this type of map)?
Is there any reason those particular points are used, they seem fairly arbitrary?
Above map is © Crown copyright (2016) Ordnance Survey. Used for private research in accordance with the principles of fair dealing as set out in the Copyright, Designs and Patents Act 1988.