Well for many reasons.  But first lets start at the beginning so that we are all on the same page. First of all the phrase "seperation of church and state" does not appear in the US Constitution.  The first amendment to the constitution allows for the right of "freedom of religion". Many SCOTUS decisions through out the past 200 years has upheld the concept of seperation of church and state.The phrase actually originated in a letter from Thomas Jefferson to clergy in Danbury Conn. I believe.

When America was just colonies, prior to 1776, many of the colonies had laws that favored one religion over another. Some more strict than others, but all of them were repressive unless you were a member of the favored church. Many of those that immigrated to the colonies flesd due to religious persecution from the church of England as well.  The founding fathers held many different religious beliefs, and some were not as religious as others.

OK enough of history. One  reason we need a separation of church and state is we do not want to create a theocracy in this country. Look at Iran if you do not understand the problems associated with theocracies.

So that is one reason, lets hear yours.

Respectfully

TJ