Clubs have to make changes in three blocks – not including half-time - in a bid to reduce time-wasting.
Often towards the end of games clubs will bring on players off the bench more as a tactic to slow the game down than to get fresh legs onto the field. This rule means that clubs can't stop the game at a convenient break point to bring on players if they've already done so three times.
If there is an injury, clubs don't get a fourth opportunity to make a substitution. Half-time is permitted as a substitution point because it isn't a chance to waste time in the match.