First there are efforts underway in court to establish that the president's twitter communications are official statements, which is important because having them so characterized has been and will be helpful in court cases such as those challenging his immigration and DACA decisions.

Second, to say that they "can't be used for that purpose" is meaningless. It wouldn't change the content of tweets but would make them informal musings that no one could rely on in challenging what the president does.

https://www.cnn.com/2017/12/28/politics/trump-tweets-legal-cases-official-statements/index.html

These are the proclamations of the President of the United States. Every bit as important as a memo or statement that they put out through other means.

They should be considered official record.