A fair number of problems in operations research, decision science, machine learning, physics, and econometrics boil down to constraint optimization. An objective function needs to be optimized, either by minimizing a cost or maximizing a reward, in relation to specific variables that have certain constraints. Real-life examples include production planning, scheduling, Sudoku, or Crossword puzzles.
Adding Constraints
Good math example of a phenomenon that I very much notice in my personal world. Constraints don't need to be viewed as a negative, but we often do.