Why do economists use real GDP rather than nominal GDP to gauge economic well being?

In the year 2017, the economy produces 100 loaves of bread that sell for $2 each. In the year 2018, the economy produces 200 loaves of bread, that sell for $3 each. Calculate nominal GDP, real GDP, and the GDP deflator for each year (Use 2017 as the base year). By what percentage does each of these three statistics rise from one year to the next?

Why is it desirable for a country to have a large GDP? Give an example of something that would raise GDP and yet be undesirable.