It's best not to emulate them
When running experiments, humans make errors. There is almost no conceivable way for a human to do anything perfectly (appart from math and similar nonphysical tasks). No matter how hard one might try, there is always a margin of error. When measuring things with a ruler, that error is approximately plus or minus one half of the smallest unit that the ruler measures to.
It's not perfect
Sources of error can come from anywhere. The most important errors to realize are the errors in your measuring device. Sure your scale might measure to the nearest thousandth of a gram, but that really isn't all that accurate. If we assume that the scale rounds, then your scale has a built in error of plus or minus five ten-thousandths of a gram. Even if you did manage to have a scale that measured every single atom (I'm not sure how that'd be possible) there would still be error in your experiment. When you have measurements that fine, slight variations in air current alone can significantly alter your measured value.
Some scales are pretty accurate
So far I've said nothing about why knowing these errors is important, so let me remedy that now. Measurements are taken for a reason, whether it is to determine the molar mass of a molecule in a compound or to determine the average velocity of an air glider. You are looking for an answer, and you will want your measurement to be as accurate as possible. If you don't do enough experiments to minimize your randome error, you are very likely to get something wrong. Can you imagine what that would be like if some lab synthesized the wrong medicine because their experiments were done poorly? Or what if a part in your car had the wrong measurement? That medicine could kill you, and that part might fail or not even work in the first place. There are numerous other examples of why you need to know your error and account for it, but suffice it to say that it's important.