Verizon customers who want to indulge in the more in-depth parts of Android don't often get the chance, thanks to the carrier's tendency to lock down bootloaders and close off most of the avenues to custom ROMs. But for major releases, manufacturers often sell contract-free variants with an unlockable bootloader. Like the S4 before it, the Galaxy S5 now has the option, and you can buy one directly from Samsung. Verizon won't sell it to customers online or in retail stores.
The 16GB Galaxy S5 Developer Edition costs $599.99 (the same as the contract-free price at Verizon) and comes in any color you want, so long as you want black. Unlike Google Play Editions, Developer Edition phones come with the same additional software as their retail counterparts - in this case, Samsung's TouchWiz on top of Android 4.4. (A stock Android Google Play version of the S5 is probably coming eventually, but not for Verizon.) The unlockable bootloader on the Developer Edition allows end users to easily flash a custom recovery, and then root or flash a custom ROM. Other than the unlockable bootloader, this Galaxy S5 is exactly the same as the standard Verizon model in both hardware and software.
Buying a Developer Edition versus a discounted, on-contract model with the intention of rooting or other modifications is a bit of a gamble. There's always the chance that someone will be able to unlock the bootloader on the retail carrier version, but that's getting less and less common these days. Of course if you intended to buy one off-contract anyway, there's no reason not to get the Developer Edition, unless you really have to have it in white.