It is not really clear why Canaletto, in a period when the demand for his paintings was dropping because of the War of the Austrian Succession (1740-48), and the big drop in tourists number doing the Grand Tour in Italy, and Venice being an important spot, started doing the etchings.
Very unlikely it must have been for financiary reasons, but there is a great possibility that his patron Giuseppe Smith, pushed him in a new direction, because he had a great interest in printmaking.
Smith may have taken the occasion - in seeing Canaletto with extra time in his hands - to push him in this new artistic adventure.
Canaletto took the possibility to put on paper some Capricci he had in his head, and never had time (or request!) to create on canvas.
On the other hand a very acclaimed etcher which was gaining his space in the Art World like Antonio Visentini (1688-1782), went straight to use some of Canaletto's more acclaimed paintings to print on paper.
Some samples here:
Etchings were more affordable (a certain number was printed) and sometimes they could come in a book, with a good quantity of images, that made the book precious as a souvenir and as an Art piece.
Etchings were made in small sizes or quite big ones, going from 14x21 cm (half of an A4 regular printer paper sheet), to quite considerable sizes - like 35x50 cm - already a good image to be quite appreciated when framed and hung on a wall, without colors, though.