If you check this setting, a new connection line "Wireless Device/sync books" will be added to the Connect menu. If you use this wireless device connection then CC will ask calibre if any book files (formats such as epub, mobi) in calibre are newer than the file already in CC. If a file in calibre is newer then calibre will resend that file to CC, replacing the book on the device. This is useful if you use the calibre or sigil book editor to change the book in some way.
Note that for reasons we don't yet understand sometimes the book files are not sent on the first connect. It seems to be related to the metadata for the book also being changed. Our experience is that the books are sent on a second connect. Thus, if you have changed book files and want them sent to CC it is best to connect, let CC stabilize, disconnect, then connect again. The second connection should be quite fast because all metadata was exchanged during the first one.