Sonneti are now creating for customers who left the rest behind
To leave the rest behind means ignoring the rest, to not show interest in something different from what you selected:
"Pick what you like and leave the rest behind"
In that context, it means:
"Sonnati products are for customers who are not happy with the other existing products and who want something different."
If you're one of the pack of frontrunners in a race, you've left the rest [of the contestants] behind. The meaning is the same here: Sonneti are targeting their products to the customers who are the trend-setters.
Previous answers suggest that customers who left the rest behind refers to discriminating connoisseurs or trend-setters. Those interpretations may be correct, but my own first impressions were morbidly different: that it references either dead persons, who have left living friends behind, or old people, whose friends have died.
Inadvertent mixing of present tense (now creating) with past (left) leads to misinterpretation. Perhaps the verb in the ending phrase should be leave rather than left.
The question was, literally, what does left the rest behind mean in this sentence?
It means, literally, that the sentence was written by someone for whom English is not his/her native language.
The confusing admixture of tenses and the ambiguous left the rest behind (the rest of whom or what? Other customers? Other similar products? Firms competing with Sonetti?) renders the sentence's meaning entirely unclear.
Yet, conversely, even though the sentence itself is inherently nonsensical, the author's intent is clear: the Sonetti firm makes products for discriminating customers.
As an aside, treating Sonetti as a plural noun (Sonetti are now making) would fly only in the UK and derivative dialects. In the U.S., Sonetti would be treated as a singular noun (Sonetti is now making), the theory being that Sonetti is a company, a firm, a single entity no matter how many people work there. If, for some obscure reason, a U.S. speaker wanted to use are with Sonetti, s/he could write Sonetti workers are now making.