Symbolism, Imagery, Allegory
Stapleton is the one who introduces us to the (fictional) bog called the Grimpen Mire. The ground is deeply unstable underfoot. As Stapleton first points it out to Watson, they see a pony trying to escape from the bog and getting pulled under (7.60).
Symbolically speaking, this mire is a place that literally holds you back. It seems to represent the general idea that Dartmoor and Baskerville Hall are caught up—stuck, even—in an older era of British history. So there's some poetic justice to the fact that Stapleton himself eventually gets dragged down into the bog. As a "throw-back […] both physical and spiritual" (13.63) to Hugo Baskerville, Stapleton represents the sordid past of the Baskerville family, now lost to the Mire. Sir Henry's continued presence at Baskerville Hall demonstrates the story's commitment to moving forward—not mired, you might say, in the past.