That's basically what counts in football especially when you are fighting head to head with other teams for the title. Barcelona currently are a very free flowing side with them being able to score as easy as possible and them struggling yesterday just shows how the leganes defense was really tight because Barcelona is one of the team currently you will play that definitely hard to defend against them.
Leganes often plays like this against strong teams, they focus entirely on defense and do it with the whole team, it is always difficult to play against such a defense, and despite the fact that Barcelona's attack is very good, they were only able to achieve a one-goal victory, but this is enough to keep their place in the table. Now Real needs to win again just to stay behind Barcelona and so that the gap does not increase, because it will be even more difficult to catch up at the end.