Why is it important to measure National Income in a country with respect to Macoeconomics?

Answer 1

Macroeconomics = the study of the performance of the overall economy
national income = Y = C+I+G+NX = consumption + investment + government spending + next exports = GDP
GDP = a measure of aggregate economic activity
hence, by studying the components, changes, effects of other variable on GDP essentially reflect the performance of the economy as a whole

Answer 2

Governments can take measures to prevent or at least minimize the effect of recessions, but only if the know when the economy is slowing down. Therefor they need to measure changes in national income.