By december 1991, the union lost all influence on its republics. If the USSR had no republics and would cease to exist after 142-H was signed, then what's the practical purpose of signing 142-H if there was no one left to recognize it as a legitimate authoritative document?
Would there be any significance if the dissolution never made it into a final document?
When the USSR collapsed, why did Gorbochev have to sign 142-H of the Supreme Soviet to dissolve the union?
Gorbachev did not sign it. He resigned as President of the USSR on December 25, the day before the Council of the Republics of the Supreme Soviet, chaired by Anuarbek Alimzhanov, adopted this declaration.
Who was the document even for?
According to Anuarbek Alimzhanov, the declaration was adopted because "...the current situation should be resolved in a civilized, legal way. Otherwise, there will be legal lawlessness in the state, serious contradictions in the Commonwealth of Independent States will become inevitable. The Council of the Republics is obliged to fully fulfill its civic and parliamentary duty and justify the trust of our sovereign republics."
The Council of the Republics also adopted other documents and decisions - on the dismissal of judges of the Supreme Court, the Supreme Arbitration Court and the collegium of the USSR Prosecutor's Office, on the dismissal of the head of the USSR State Bank and his first deputy.
Source: Brzezinski, Zbigniew. Russia and the Commonwealth of Independent States: Documents, Data, and Analysis, 1997.