The American cause was one of hardship and struggle, early in the war. A fledgling army fighting one of the most well-trained fighting forces in the world. From an outsider perspective, especially on a global stage, any country seeking to support the Americans had to see something that would show an investment would be worthwhile.
For the French, this was the American victory at Saratoga in 1777. General Horatio Gates forced the surrender of over 6,000 British forces, led by General John Burgoyne. This was a huge, crushing defeat for the British. It was also a timely and significant sign that the Americans could hold their own in this war, and support could help tip their scales towards victory.
Waiting on news of a significant victory, Ben Franklin was already poised in France under diplomatic intentions. While France (and their ally, Spain) had been unofficially helping the American cause, they now entered into a formal alliance with the Americans. Spain, being formally allied with France, would in turn help support Frances new ally, America, in the war.
This was a major turning point, the Americans secured a major victory on their own, their diplomatic chops were being tested, and served them well, and now an international force was behind their fight for independence.
Were there other factors at play that influenced the French aid? Was there anything else the French were looking for before they would openly support the Americans?