Which spurred the immediate question: Why not just have two accounts on one phone? Who doesn't have that?
For one thing, it's important to remember that the technology — along with expectations from IT departments — was not in the same place in 2009. Your iPhone or Android can handle multiple e-mail accounts easily, but even back then, the iPhone hadn't figured out how to have a unified inbox. The conservative group America Rising has pointed out that Clinton talked about having two phones in an interview earlier this year — but that was this year.
When President Obama took office in 2009, there was a lot of media attention paid to his fight to keep his BlackBerry. He won, as presidents probably tend to do; the deal worked out allowed him to "stay in touch with senior staff and a small group of personal friends in a way that will be limited and that the security is enhanced to ensure his ability to communicate," according to then-spokesman Robert Gibbs. (Emphasis added is mine.) He had been using two phones: A BlackBerry and a Sectera Edge, made by General Dynamics, that plugged into the BlackBerry. (Here's more on the Sectera, if you're curious. And if you're wondering, Obama still uses a BlackBerry.)
Obama's former speechwriter Jon Favreau indicated on Twitter that two devices was the norm. "[Y]ou couldn't [have two accounts on one phone] with WH accounts (and I guess State too) until maybe 2011-12," he tweeted, later adding that "eventually there was an app for your iPhone that allowed you to access your WH emails," and that "you couldn't access Gmail on any WH computers either." Senior administration officials having two phones is still common to this day.
Others offer a different story. BuzzFeed's Evan McMorris-Santoro reported that when he was transportation secretary, Ray LaHood had both government and personal e-mail on one BlackBerry device. Emily Miller, a journalist who was an appointee in the State Department under President George W. Bush, indicated the same, tweeting that "we had both unclassified State email and personal email on the same Blackberry" when she was in the administration.
The word "unclassified" there is important. The government uses two networks, SIPRNET and NIPRNET, the former of which is used for classified communication. A report just last year indicated that the Defense Department was still working out how to allow SIPR access on a bring-your-own-device basis (meaning, not on a government phone); it wasn't until the end of 2013 that it had a system allowing remote SIPR access from desktop devices. (As is the case with a lot of nuanced government decision-making, there are murky areas. Know something? E-mail!)
Update: A document provided to The Post from Clinton staff indicates that none of the e-mail sent to Clinton's personal account was classified. "Classified information was viewed in hard copy by the Secretary while in the office," it says. "While on travel, the Department had rigorous protocols for her and traveling staff to receive and transmit information of all types."
The standard appears to have been (and in some cases still is): a secure phone provided by the government plus an unsecure phone for your personal use. As with all things government, it's not clear yet how uniformly this was applied, including across administrations. But it does suggest that the question being asked shouldn't be "Why would Hillary Clinton need two phones?" and should be instead "How secure was the phone she was using?"