The main meaning of sending a lawyer's letter is: the other party's certain product has infringed on your XX patent, please ask the other party to immediately stop the infringement and compensate for the infringement that has occurred.
The role of the lawyer's letter is to keep it as evidence when suing. In addition to the lawyer's letter, there are other ways to leave a record of the contents. For example, you can also send a telegram, because the contents of the telegram will be left to you as a stub.