A good read from this web site.
http://antiqueradios.com/superhet/
Quote. Paragraph from the page.
Heterodyne detection provided an apparent amplification of the received signal, an important effect since at first no other method of radio-frequency amplification was known (the Audion was used only as a detector for several years after its 1906 invention, not as an audio- or radio-freqency amplifier).