1. Discover / browse / search.
2. Expect high quality text & typography.
3. Be part of a high-volume community that archives and refines texts year after year.
"Then build your own similar site with high-quality titles in French!" , I hear some say. Sure, but I see tons of excellent marketing & infrastructure already done by Standard Ebooks, which could be reused for non-English books!
Said differently, mid/long term I see the greater good being achieved by supporting non-English titles in Standard Ebooks itself, not in one satellite mimic site per language, of varying maintenance quality. To compare with the domain of online encyclopedias, these are the same reasons to have en.wikipedia.org and fr.wikipedia.org maintained under one (Wikipedia) umbrella sharing infrastructure, not wikipedia.org and frenchwikipedia.org maintained by entirely different teams.