The legal theory is that Paul Manafort was the nexus around which possible collusion could take place. The hope being that he will provide evidence against other conspirators (e.g. the Podestas) in exchange for a reduction in these charges.
If Robert Mueller left these charges to someone else, they might not be ready when he needs them as leverage against Manafort. Also, as he is investigating this, he can subpoena records from Manafort based on this investigation that his election investigation might not be able to request.
Of course, the risk is that if Mueller can't convict on these charges, his leverage to turn Manafort against other conspirators goes away. And he might lose the ability to use the evidence gathered for this investigation in other crimes. It is presumably for those reasons that Mueller has been reluctant to move forward with this step, as it has been rumored for months.
Some have asserted that there is no link between Manafort and the Podestas. I posted some links here and in a comment. It was then asserted that those links don't connect Manafort and the Podestas. I think this is ranging off subject, but apparently that's where people's minds go. Anyway, you can see for yourself:
That's the overall takeaway from a series of news reports, including one from NBC that indicated that special prosecutor Robert Mueller has been investigating the business dealings of Democratic lobbyist Tony Podesta. His firm, the Podesta Group, is one of several that did work on behalf of Paul Manafort, the former campaign chair of President Donald Trump's campaign.
The Podesta Group, a Washington lobbying and public relations company known for its ties to Democrats, is among several firms that worked with former Trump campaign chairman Paul Manafort on a multi-year effort to boost the standing Viktor Yanukovych, the pro-Russian president of Ukraine who was later forced from office by a 2014 uprising. Tony Podesta is the brother of Hillary Clinton’s former campaign chairman, John Podesta.
The probe of Podesta and his Democratic-leaning lobbying firm grew out of Mueller's inquiry into the finances of former Trump campaign chairman Paul Manafort, according to the sources.
Salon and Mother Jones are left-leaning news sites. NBC News is more establishment, although it's worth noting its connection to MSNBC, the most liberal of the cable news networks.
The Podesta Group was founded by brothers Tony and John Podesta. John has spent a good portion of his career working for Hillary Clinton, most recently as her campaign manager.
It's interesting to see how the disinformation flows. For example, a commenter says:
How could he provide evidence on the Clinton campaign, when he worked on the Trump campaign?
He presumably being Manafort in that context. Sure, it is unlikely that Manafort had information on the Clinton campaign, but that's a strawman. He may very well have known about other episodes of Podesta collusion with the Russians, e.g. the Uranium One deal. From the New York Daily News:
Records at opensecrets.org show that the lobbying group also received $90,000 from Uranium One, a company that controls U.S. uranium interests. The company’s sale to Russia has drawn scrutiny because of Russian interests donating to the Clinton Foundation.
Again, the NY Daily News is a left-leaning paper.
The point being that this was the use of a small truth to mislead.
All this is interesting because the predominant narrative to this point has been the possibility that Manafort knows something about Trump colluding with the Russians during the election. But Manafort has much more history with people other than Trump. So turning him may not reach Trump but might instead go through Tony and John Podesta to Hillary Clinton. And there's already substantial evidence that her campaign paid (Vox is another left leaning news site) $9 million to (indirectly) Russian sources for opposition research on Trump.
You know, if you scream "Right wing conspiracy!" every time something negative comes out about your candidate, you are eventually going to be proven wrong. It's just too bad that today was that day. There certainly is a link between Manafort and Tony Podesta, just as there is between Manafort and Trump. Whether those links eventually cause additional indictments is entirely speculation. It's noteworthy that the current charges link to lobbying (what the Podesta Group does) and not to the election (Manafort's connection to Trump), but we don't know where they might go in the future.