I bought 1 book using Kobo Wifi from Kobo on line store as a test and a few more using Kobo Desktop program. Discover, Wishlist, and other book sales promotion on the reader or desktop program I don't mind if they don't use a lot of processing time or memory and now are not really intrusive.
What I find useful is Feedbooks has short author biography in front of their books and lists other books by the author with download links. At back of book is short introductions to other books by author with download links. Paper back books had this for years and I have down loaded ebooks from these links but the the links are not accessible from Kobo readers. This is probably because they bypass kobo store but are enticing advertising because I'm looking for another book to read.