Whether you use the English term Outbound Clearance or the Spanish term Zarpe. A document showing you left one port free in clear is best to have before you try to enter another port. The US does not make US flagged vessels go through any specific procedure to leave. However many jurisdictions will not let you in if you cannot prove you left your previous port with the exact passenger list in good standing. Noonsite and other cruising forums detail families with boats confiscated in Mexico or Americans in the pacific sailing hundreds of miles back upwind to Haiwai to get a clearance document they failed to pick up on the way out.

I guess you could look at it this way. Do not go further than you are willing to back track without the proper document saying you, your crew, and your boat left in good standing from the previous port.

The document is basically telling the next port captain you left without running out on any bills, or fines, no one on your crew list was wanted for a crime or prohibited from leaving the previous port for some reason. All good things to know if you are the immigration officer in charge.