Yeast pitching rate is vitally important to getting the proper flavor out of your beers. Its been said that yeast can produce up to 500 distinct flavors in your beer that are for the most part entirely dependent on pitch rates and fermentation temperature.

Under pitch and you make your yeast work TOO hard resulting in soapy, chemical or possibly even corn flavors. 

Over pitch and you will lose some of the esters and flavors that the yeast add to your beer because when given the opportunity  are lazy and don't have to do as much work.

Just like Hops, grains and adjuncts should be measured more or less exactly, your yeast additions so should be properly measured. Use yeast calculators and make the appropriate sized starter and pitch the appropriate amount of yeast to do the job and do it right.

If you under pitch your yeast may not even finish fermenting the beer because its been put through so much stress already.

If you don't really care are happy drinking whatever your fermenter spits out you can ignore pitching rates and get beer, but if you want to perfect and make high quality homebrew you should really monitor your pitching rates.