why is it better to use serial dilutions?

Often in a lab you will have a stock solution rather than the raw ingredients.  It's faster and allows more people to make the dilutions they need from one stock.  

Also, for some chemicals, the amount you'd need for a low concentration cannot be measured on the scale you have in your lab because the chemical powder is too light and you need so little of it.   I once worked in a lab where the only scale we had measured in grams to one decimal place (0.0g), but I needed something specific like 352mg (0.352g).  Didn't have time to buy a new scale, so dilutions were used.