When we play against poorer teams that aren't good and will only sit back and counter, we can afford to push the 8's up and attack more. We dominate possession and chances because they aren't good at holding the ball themselves. Against better teams Pep will usually play Gundo with Rodri because he wants ball control. He needs both of them to play out from the back trying to be more in a 4-2-3-1 against dangerous teams that press well and are good with the ball like Real Madrid for example.
It can stifle our attack more and you can criticize Pep if you want for this, but he wants to control the game as much as possible. Ball control=more possession=the opposition getting fewer chances. It can backfire and he can be overly cautious at times, but Gundo also has the ability to be a deep lying playmaker from a 4-2-3-1. There are some games in which he wants Gundo to be both 6 and a 8. If he sees we dominating a team in their half Gundo can go forward and be an 8 or get into the box himself. Of course he's not the best 8, but I believe this is why he gets chosen more then people would like.