Why is the boston tea party so important

The Boston Tea Party was important because it showed the American colonies had grown tired of arbitrary taxation by the British. When the East Indian company was allowed to sell tea directly to America, the British instead the the tax be paid on it. This culminated in the dumping of tea cargoes into Boston Harbor. 

The topic of the essay:  a. the purpose of government.

I won't write your essay for you, but as a Teacher can provide some advice as you prepare to write.  Lincoln's assertion is that the proper purpose of government is do for the people of a nation or community what individuals could not do very well if they each tried to do it themselves.  You might think of examples like infrastructure -- road building, ports, public utilities -- as matters that need community to come together rather than "every man for himself."  Similarly, matters of public safety like police protection in cities or national protection through a military are better done by a government working as a cooperative entity of the citizenry, rather than expecting each individual to provide all of his own protection and security.  There could be many other examples (including a contested one, such as health care) that could be used to give support in agreement with Lincoln's argument.