Compactness over territory
A low block sits deep and narrow, inviting the opposition to have the ball in areas of the pitch that carry little immediate danger. The trade being made is territory for compactness: rather than spreading across the whole pitch to contest every zone, the defending team compresses into a small central area near its own goal, making it hard to find gaps between lines. This looks passive on television because the ball spends long periods far from goal, but the organisation required to hold the shape without panicking is considerable, and it rewards patience rather than aggression.
Why teams choose it deliberately
Sides with less possession often choose a low block not because they lack ambition but because it is the version of defending that best suits players who are quicker in reaction than in anticipation. Standing off an attacker rather than engaging early avoids being turned or dribbled past in space, and it keeps defenders goal-side at all times. It is particularly effective against teams that push full-backs high and commit midfielders forward, because a compact low block leaves little room in behind for the kind of quick combination play those approaches depend on.
The discipline it demands
The hardest part of a low block is resisting the instinct to press the ball when it finally arrives nearby. A single player breaking the line to close down an opponent can open the exact gap the whole structure was built to prevent, so successful low blocks are built on trust that a teammate will deal with the next pass rather than everyone reacting to the ball at once. When it works, it is not stubbornness; it is a considered judgement about where a team is strongest and where it is willing to concede space.