Sappers' Bothy is the (ruined) 19th century bothy built near the summit of Ben Macdui. The use of 'bothy' in the name correctly implies the purpose of the building ; to house men while engaged in their work.
In 1847 Sappers (a slang term for Royal Engineers) occupied the summit of Ben Macdui while surveying the Cairngorms. In Burton (1864) the author recounts finding the sappers at work on the summit of Ben Nevis – giving us an idea of the scene on the summit of Ben Macdui in 1847 – writing :
a crowd of soldiers, occupying nearly the whole table-land of the summit … with their red coats, dark-grey trousers, and fatigue caps … The party were occupied in erecting a sort of dwelling for themselves – half tent, half hut. Though in fatigue dresses, and far from being very trim, it was easy to see that they were not common soldiers. They belonged to the educated corps of sappers and miners … They had already conducted some operations on Ben Muich Dhui, and they were now commencing such surveys on Ben Nevis as would enable them finally to decide which of these mountains has the honour of being the highest land in the United Kingdom
– Burton (1864) (p11, 12, 14)
Their ‘operations’ are commemorated in the name Sappers’ Bothy – the ruined “half tent, half hut” a few hundred yards south-east from the summit. I don't yet know what these operations were, but apparently the Sappers occupied their bothy for at least a few summer weeks - in Gordon (1925) the author writes :
A few hundred yards from the hill-top are the ruins of a bothy where the sappers who surveyed the Cairngorms passed a summer sixty years ago. Their visit to Mar is even now remembered, for they paid the crofters of Inverey handsomely. The crofters on their ponies carried up coal and provisions to this dwelling on the roof of Scotland 4,000 feet above the see – almost as high as the observatory on the crest of Ben Nevis
– Gordon (1925)