From the point of view of history, the Transcendentalist movement is important for what it tells us about the time in which it arose. The Transcendentalist movement arose during a time in US history often referred to as the market revolution. This was a time when the US economy was moving away from a system in which people grew and made what they needed or, at the most, traded with neighbors for them and towards a system in which people sold things for money and used the money to buy other things that they needed.
This move towards a market or commercial economy was accompanied by major changes to society. Society came to be more individualized and atomized. Because people did not farm for themselves any more, they lost some of their independence. Because they did not trade with neighbors, they lost some of the social structure that had held their society together.
Transcendentalism is seen as a reaction to these changes. The Transcendentalist emphasis on nonconformity and individualism, for example, is seen as a reaction to the loss of independence that came with working in a commercial/market economy. The emphasis on being one with nature and with all of creation can be seen as a reaction to the loss of community and the feeling of loneliness that came with it.
To historians, Transcendentalism is important because it is said to tell us something about the state of mind that was caused by the changes of the market revolution.