Yes, you need something in order to install a drive in the area where your optical drive is. You won't really be able to do it without one.
You can obviously buy a knock off optibay at a lower price, I did when I bought my first one, and for the most part it does the job it says that it will do. It was a cheap plastic little skeleton of a mount and did the job it said it would do.
Later on I purchased a newer computer not too long ago, I decided to buy the actual official optibay.
Now, I am someone who likes build quality, and the official optibay was superior to the knockoff one that I purchased before that. It is light and wasn't just a plastic skeleton that held the drive like the knock off. It was a full thin metal chassis that seems like it would hold it more solidly and not jitter around. It was made of thin sturdy metal and felt more rigid, like it was going to last. Also, when I purchased mine, it came with a free DVD drive enclosure, which works great and it didn't require me to go and acquire one somewhere else. I was able to keep the DVD drive and it even works on my other macs. Pretty convenient but not entirely necessary. The enclosure was very bare bones and was plastic but it was a nice gesture and it works. The Optibay is now only 50 dollars, when I bought it, it was about 100 with shipping and handling but came with the enclosure for free. The enclosure is now a 30 dollar add on.
The bottom line is that though there is a difference between knockoffs and the official one, whether or not you will really care about those difference is entirely up to you. The experience that I received with the official was great and it is backed by a company that actually cares about their customers and they test all the new laptops to make sure that everything will continue to work. If you end up buying a knock, just make sure that it will work with your exact make and model and year of your macbook pro. Don't usually expect support, when I bought my first one, it was off of ebay and really cheap and I can't even remember the company it came from. Also, be sure to check the size limitations, the official optibay supports drives that are 9.5 mm thick, others may give you slightly more space or be slightly different since their are multiple knock offs out there.
Just read reviews and research, you'll find the one that you like. Also, make sure that you also research about SSD in your computer and things that you can do to optimize things, it never hurts to learn as much as you possibly can.