why was this novel so important when they wrote it

Pudd'nhead Wilson was written during Mark Twain's "pessimistic period." At the time, Twain was living in Italy, attempting to recover from his recent bankruptcy. 

In Pudd'nhead Wilson, Twain delivers a scathing critique of slavery and race relations in the American South. He highlights the arbitrariness of racial classifications and distinctions, by showing how easily Roxy (a slave) is able to switch her own child with her Master's offspring. The young, usurping slave grows up amongst whites without drawing any suspicions. Twain thus demonstrates how artificial and constructed racial distinctions truly are. The themes of this book are still relevant to many aspects of life today. Check them out at the GradeSaver link below:

http://www.gradesaver.com/puddnhead-wilson/study-guide/themes