I have 6 decks that I carry: 2 in the bag, and 4 new in the car. I used to work at a hospital and at my local elementary, and I used to entertain the kids with card tricks. The two I carry are red and blue decks, because having different colors open up a lot of opportunities for different card manipulations. The other two I keep as backup are also red and blue, because I often end up marking or destroying the deck during or before the trick.
I prefer Bicycle over other brands because it shuffles very well out of the box, and is very consistent in texture and thickness. The air cushion and gloss angle also create perfect traction for fanning and counting and weaving. Knowing exactly how a deck feels in your hand before you open the shrink wrapping is important if your trick relies you on knowing exact placement of certain cards. Also, if you're doing deck ripping to impressive people, Bicycle cards will almost never have defects to prevent a clean tear. Other brands could have inconsistent cuts or paper thickness that prevent you from smooth out-of-the-box handling or doing a clean rip. Also, Bicycle's backing are super convenient for homemade marked decks.
Card games are good icebreakers, too. It makes waiting for food a lot less awkward if you're not a smooth conversationalist on first/second dates. You can kind of tell a lot about people from the way they behave in a competitive card game. If you don't know how to play any good 2-player games, you should learn crazy 8s and rummy. Rummy is more fun because the games are shorter, but it requires some level of knowledge of poker, which not everyone has. Crazy 8s is classic, and can be taught in just a few sentences.
I've used Tally Ho, which is what other people also like, but I decided to just keep it simple and keep it to one brand. I must've opened at least 500-600 decks by now, and I have never, EVER found a blemish in Bicycles. If you don't plan on cycling through multiple decks over a short period of time, you can get one of those weather-resistant all-plastic ones. They don't shuffle that well and they're often too narrow to handle in your hands comfortably, but they're good for kitchen-table card games because you can spill soda/beer on them and you can just wash it off.
TL;DR Because why not.