Including irrelevant costs can:

$1.$ Overestimate Costs: By adding things like fixed or sunk costs, you might make the purchase seem too expensive.

$2.$ Reduce Future Profits: You might pass up good opportunities because your cost estimates are too high.

$3.$ Lead to Bad Decisions: You could make less efficient choices, increasing future expenses.

Focusing only on relevant costs helps companies make better decisions and improve future profitability.