There is no single correct time, and the bar is designed as a flexible snack rather than a meal replacement. The source information suggests several practical situations: a small bite before or after exercise, a backup option kept in a bag or desk drawer, and a more protein-focused alternative to biscuits and candy when you want something sweet with coffee or tea.
If you eat it before training, the main benefit is having some fuel and protein in a compact form. If you eat it after training, keep in mind that 10 g of protein is a modest amount; it can be part of a snack, but if your goal is a larger post-exercise protein intake, you may want to combine it with other protein-containing foods. Between meals, the protein and fibre can help you feel fuller for longer than a conventional sugar-based sweet, since protein is digested more slowly than sugar alone.
It is also convenient when you need a shelf-stable snack that does not require preparation. The same bar works equally well at midday, in the afternoon, or as a small evening treat. What matters is how it fits into your overall daily intake, not the clock.