What is the reason to use OpenJDK?

Question

I somehow understand the differences between Oracle JDK and Open JDK.
But I can't find reasons to use Open JDK.
Because Oracle JDK already has everything one would need and sometimes faster than Open JDK.

In which cases should someone use Open JDK?
Is it just for legality issues?
If yes, then what are issues?
That because I have been still develop all my projects using Oracle JDK

Answer 1

Other Answers are correct. Here is some other information as well, including vital changes as of early 2021.

Source-code versus Binaries/installers

understand the differences between Oracle JDK and Open JDK.

To sum it up: source-code versus binaries/installers.

• OpenJDK is an open-source project, implementing the Java Specifications, JSRs, and JEPs that define the Java platform. This project publishes only source-code, not binaries or installers to get Java running on your computer (except for some early-access builds). Oracle is the principal participant in the OpenJDK project, alongside IBM & Red Hat, Apple, SAP, Azul Systems, Microsoft, and others.
• Many companies provide distributions of OpenJDK for installation, as binaries or installers. Oracle is one such company.

Oracle actually provides two such products:

• Oracle JDK
  A commercial product, with paid support plans. (Free-of-cost only for development & testing, not deployment. Read their terms.)
• jdk.java.net
  A build of the OpenJDK source-code, freely available, unsupported.

Oracle has declared their intention to keep their commercial product Oracle JDK at feature-parity with OpenJDK. Oracle even went so far as to open-source and make available at no cost their formerly commercial tools Mission Control and Flight Recorder, both now a part of OpenJDK. So there is nearly no practical difference. One significant difference is that the Oracle company reserves the right to rapidly supply their paid support customer base with urgent patches not currently found in OpenJDK. Ditto for other vendors selling support.

[And for the record, Oracle offers a third implementation of Java, GraalVM, which is a very specialized product.]

Here is a flowchart graphic I made to help guide you in selecting a provider of a Java implementation.

And here is a list of motivations you might consider in choosing a distribution.