A provisional patent application does not become an issued patent. It actually just sit in the USPTO without being examined for potential issuance. Then, what a provisional patent application do?
It basically earns time to decide whether you should go forward with obtaining patent for an invention.

How it goes?

You basically disclose your invention in writing, send the document to the USPTO where it will be time-stamped, and your priority date for establishing “who invented first” is valid for a year.

If you file a non-provisional application within the year, your application will undergo examination and eventually can get issued to become a patent.

Why it matters?

Well, you usually don’t know if your invention is going to be something you can and want to profit from at the time you come up with your invention. Provisional applications require lower cost to prepare (by lawyers) and file with the USPTO compared to non-provisionals. Hence, you save money while preventing others from taking your idea away.

In practice,

It could be a routine practice to file a provisional first, followed by a non-provisional in an almost full year. It just makes sense to be cautious before incurring a significant financial burden. However, in some cases, an inventor may intentionally delay the examination of the patent by filing provisionals because they believe the application has very low likelihood to become an issued patent.

Lastly,

As soon as you file a provisional application, your invention is “Patent Pending” just like non-provisional applications. It usually means that you can start disclosing and selling your invention without worries about someone running to the USPTO to steal your idea!