The Canon BG-E14 Battery Grip was specifically made to go with the Canon EOS 70D. The E14 comes from a long line of Battery Grips that have improved over the years, like most other Canon accessories that are based on technology.
For the majority of photographers, I believe the "third party" grips are a better choice. Most of the third party manufactures do a good job of making sure that these grips are fully compatible and they are a fraction of the cost of a genuine Canon grip.
For loyal Canon shooters and professional photographers who want the absolute best function and feel to their equipment, the genuine Canon BG-E14 is the superior choice.
If you are new to the idea of what battery grips are for, there are 3 main advantages of using one of these accessories.
You essentially double your battery capacity with the insertion of two freshly charged batteries in a grip and add the ability to use 6 AA batteries as an alternative. If you are shooting video or using your LCD screen extensively, the extra power capacity will be appreciated.
Using the GPS and WiFi capabilities also wear down batteries quicker.
A more ergonomic position for your hand to securely grip and control your settings for less arm and wrist fatigue. It takes little getting used to if you are accustomed to using the regular camera controls on your 70D.
Many photographers who've purchased a battery grip and used it fairly extensively don't want to use a camera without one.
This is more of a subjective preference than a functional difference. Long lenses can make a DSLR feel front-heavy and awkward to hold. Adding the size and weight of a battery grip attached to the base of the camera distributes the weight more evenly.
For many photographers, this is the way to get the same balance you get with one of the Canon 1Dx series of pro DSLRs.
The Canon BG-E14 Battery Grip has full system compatibility with the Canon EOS 70D Camera. It gives the photographer improved handling and versatile control. Extended battery life is the most obvious benefit to having one of these accessories.
The BG-E14 accepts either one or two LP-E6 battery packs. Two battery packs will double the camera's battery life. You can also use six AA batteries or the ACK-E6 AC adapter kit for continuous power using electricity.
The grip's design makes shooting with a vertical orientation easier as well as making it more comfortable to hold the 70D. The grip is integrated with multi-function, AE/FE lock button, AF point selection button, main dial and shutter release buttons.
The Canon BG-E14 Battery Grip is compatible with the Canon EOS 70D DSLR Camera and provides improved handling, versatile control, and extended battery life when in use. It accepts up to two LP-E6 lithium-ion battery packs to effectively double the camera's battery life. Six AA batteries can also be used as well as the optional ACK-E6 AC Adapter Kit for continuous power.The design of the grip improves shooting in the vertical orientation and also provides an overall more ergonomic structure for holding the 70D. A second shutter release button, main dial, multi-controller, AF point selection button, AE/FE lock button, AF start button, and multi-function button are all integrated into the grip for improved and versatile handling.