A centre-back leaving the defensive line to challenge a midfielder looks like a gamble. It is one of the more organised actions in modern defending, and it is triggered by a specific situation.
The space between the lines is the problem being solved
A player receiving between the opposition midfield and defence is facing goal with time, which is the most dangerous position available in open play.
Midfielders tracking backwards arrive behind that player and cannot intervene without fouling, so the defensive line is the only unit positioned to act.
Stepping forward denies the reception entirely, which is a cheaper solution than dealing with the consequences of allowing it.
Timing is set by the pass, not by the opponent's position
The step is made as the pass is played, when the receiver is committed to receiving and cannot spin away before the ball arrives.
Moving earlier tells the passer the space is available behind, and he simply plays a different ball into it.
This is why the action requires a defender who reads the passer rather than watching the man he is marking.
The vacated space has to be filled by someone
A centre-back stepping out leaves a gap in the line, and an opponent running into it beats the whole defence at once.
Teams cover this with an agreed rotation, usually the remaining centre-back shifting across and the far full-back tucking in to reform a line of three.
The rotation is rehearsed until it is automatic, because it must happen while everyone is moving and nobody has time to decide.
Front-foot defending suits a high line
A team defending with a high line already compresses the space between its units, which shortens the distance a centre-back has to travel to intervene.
A deep block has more space in front of the defence and less behind it, which changes the trade-off and makes stepping out less attractive.
The behaviour is therefore a property of the system rather than of the individual, and the same defender will step out at one club and hold the line at another.
Losing the duel is the accepted cost
A centre-back who steps out and is beaten leaves an attack running at a defence one player short.
Coaches accept this because the alternative concedes the reception every time, and the occasional loss is cheaper than the repeated concession.
What is not accepted is the half-measure, where the defender steps out without committing and arrives too late to challenge but too far forward to hold the line.