Hello, friend. Welcome to the league! As someone who plays bots regularly myself, let me enlighten you.
People play bots for a variety of reasons. The most obvious one is to practice the game, but people also play them for:
*Learning new champions or roles. Bot games are great for people who don't feel comfortable going against others with a particular champion or position. I myself am learning to jungle with Vi and Shyvana, and am not confident enough in my skills to take the role in a real game.
*Trying out new item builds. Again, because of lack of comfort doing it in a real game. If the build works, awesome. If not, you learned how NOT to play this champion. The game mode is insanely relaxed and your team mates don't care. It's perfect for experimenting.
*They're quick. In bots you can go the entire game without killing one jungle monster. People don't care as much about dragon or baron, the pressure is all on the lanes. In PVP you've got people everywhere watching your moves and trying to outplay you. All the bots can do is spam abilities and buy items. They're intelligent, but there's no real strategy.
*The first win of the day bonus. People turn to bots because winning is much easier than winning a PVP game. They can play their one game and be done if they so wish.
One last thing I do want to mention. Experienced players can, of course, play intro/beginner bots if they want. HOWEVER, it's not smart to do if you're playing for for the IP. Riot purposely makes the IP earn ridiculously low if you're higher level playing intro/beginner. Be aware of your level and Riot's policies regarding this so you can get the most from your first win bonus.
Good luck on the Fields of Justice!