Hillary needs support with election critical Global warming Science.
Who's better able to lend support than Al Gore, who's expertise besides Global Warming is a claim of fame by having invented the Internet.
There is precedent for a former vice president running for the office again: Adlai Stevenson (grandfather of the Democratic nominee in 1952 and 1956) was Grover Cleveland's VP in his second term, and ran again in 1900, and Charles Fairbanks, Theodore Roosevelt's VP, ran again in 1916. Neither had run for President in the meantime.
If the Democrats regain the House in November and make a big to-do over impeaching President Bush, would that hurt Hillary? It could remind people of the impeachment of Bill Clinton and by extension of the multitude of Clinton scandals.