on what instances where serial dilution is important.?

Answer 1

When you're diluting by a very large factor. For example, if you're doing a 1:1,000,000 dilution, you don't want to add 1 microliter to a liter, because you're likely to get big errors. Instead if you add 10 microliters to 1 mL (100x dilution) three times, your cumulative error is likely to be smaller, just because measuring out really really small volumes very precisely and accurately is hard.