why we use first voided morning urine sample for test?

It's a concentrated read on what's going on with your body's metabolism, overnight night without the interference of excreted nutrients from foods.

That's best for a pregnancy test because the concentration of HCG is predictably going to be higher, most people not making a habit of drinking while they're asleep.

Because that urine is more concentrated than any thru the day.

It ensures that nothing you have recently eaten or drunk will drastically influence the results, as it could lead to a false positive or false negative.