What is the importance of Joule's experiment?

Question

I was reading about the experiment of Joule (Italian wiki page). I'm not sure how it's called in English, since there is only an Italian and French version. In any case, in the page it is stated that (my translation):

Through this experiment, Joule determined the mechanical equivalent of heat to be equal to 4,186 J/cal, value of extraordinary precision for the time.

Is the importance of this experiment "only" ascribable to the fact that the evaluation was really precise and exact for the time? Or are there other reasons?

I don't need a whole list of reasons, just the most exemplary/important¹ fact(s) will suffice.

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^{1: Objectively considered as such by most researchers, experts, etc.}

Answer 1

The answer can be found in the Wikipedia page you linked to! Historically, heat had been considered a substance, called caloric. Joule's experiment proved that heat was actually a form of mechanical energy, so was a crucial step towards our modern understanding of the conservation of energy.