Why do i need to set 'Content selection mode' to 'All content' in i18nsettings ( admin/settings/i18n )

On the module page i read 'Compatibility':
"If you have i18n, you should set 'Content selection mode' to 'All content' in i18nsettings ( admin/settings/i18n )"

I did this.. module works perfect (for me)
but now i changed it back to:
Mixed current language (if available) or default language (if not) and language neutral.
(to test and get my default frontpage with this settings, of course i can create a view for frontpage but i wanted to test)

and the module still works perfect! So i can i keep settings like this.. or better to set it to 'All content' and create frontpage view with content "select translation" Filter.

And what will happen if i use this i18n setting?