What was the historical significance of the McMahon Line?

The McMahon Line is an imaginary border between the British Empire and Tibet in the early 20th century. While Tibet was conquered by China in the 18th century, the British Empire later conquered the northeast Indian states of Assam from the Ahom Kingdom which earlier bordered the Kingdom of Tawang. In addition to the historical ties between Tibet and Tawang, the other reason that Tibet claims Tawang is that Tibet supposedly still holds suzerainty over Tawang while the British Empire turned it into a protectorate. This led to the McMahon Line, marking the border between the British Empire and Tibet which includes the state of Tawang to the British Empire, marking the border between the two. McMahon Line remained obscure until India inherited the entire British Indian Empire. McMahon Line is well known today because China claims Tawang as well as many territories that are to the south of the McMahon Line.