Eh, I wanted gator to actually answer. Killjoy!
However to answer your rhetorical question: So the Democrats can keep the farce going in their nonsensical bid to delegitimize the sitting administration.
Whitaker is going to very quickly end the 'investigation', either by forcing Muller to submit his final report immediately, outright firing Mueller, or simply cutting off his funding. Whitaker can take the political heat for either of these options, since he's not likely going to be the choice to be Sessions' successor. This terrifies the Democrats, who are planning on spending the next two years making themselves look stupid with frivolity like the Russia collusion nonsense.
Just look at the histrionics by Schumer. He wants to "protect" Mueller by inserting legislation into the next 'must pass' spending bill. Apparently he didn't learn from the last shutdown that he isn't going to win that game against Trump.