You don't have to use the Metric measurement system instead of Imperial measurement system, but there are some clear advantages.
First and foremost is the fact that there are no fractions, and little or no need for decimals (1 mm is equal to .03937"). Measuring for a plumbing hole in the back of a sink cabinet never requires your employee to know how to find a common denominator, or have the conversions from fractions to decimals memorized, they are working with whole numbers.
Your employees never need to calculate what half of 21-9/16" is, they just need to calculate half of 548.
Simply stated, the Metric measuring system sets your employees up for success, and removes many opportunities for mistakes, and although they may not be familiar with Metric, the learning curve is exponentially shorter (some employees will never know how to find a common denominator, no matter how much time you spend with them).