French alliance why? why so important?

I'm assuming you mean the French alliance with the American colonists during the American War for Independence against the British.

The French alliance was hugely important for two reasons:

France provided significant military support, especially in leadership roles like that of General Lafayette, and in support provided by the French navy in battling British ships and transporting reinforcements for the American patriots.France devoted enormous financial aid to the Americans. The cost to France for supporting America’s revolution added up to 1 billion livres  (about 4 billion in today’s dollars).  That financial boost was much needed by the revolutionaries in America.  [It did also have the side effect of putting the French government deeper in debt, which led to the French Revolution.]