i dont pretend to be an tactical expert but i think it all depends on situation tbh. for example if you have a slow defence you should generally play a fairly deep line (as england found out) although this does not apply in all situations. factors are opposition, pitch length, ability of team, phsical attributes of team amongst many other things. barcelona would play a high d-line because they want to pressure the opposition and win it high up the pitch. also a slow tempo would suit them because the oppsition are likely to be set up defensively, so they have to patiently pass around and work an opening. and as for narrow width they may employ this so that the wide players are more involved in the centre of the pitch where their creativity is put to better use. they rely on alves and abidal to bomb forward and provide their width for them. like i say i dont pretend to be some sort of tactical messiah, this is just how i would see it