When you draw from life habitually you begin to identify and empathize with your subjects in a way you can only truly understand by experiencing it yourself. You become them. Please, don't just read this post and say to yourself, "yeah, I should draw from life more" - go do it! There's nothing more for you to learn here! The real learning happens when you go do it. All I can do is to try to convince you to do it.
I could show you some drawings - here are some I did on one of my many trips to a local farm.
Down on the farm
Maybe they're not very impressive. Neither is listening to a violinist practice scales, or watching a ballet dancer stretch at the bar. What's happening in your brain, though, is pretty significant - and it comes out later in all the work you create. Drawing what you see around you should become a compulsion, like someone who is blind feeling people's faces in order to "meet" them, or like when you hear music and can't help tapping your feet and singing along.
Hanging out at the pool
With practice you begin to melt into the subject and the process. Eventually it feels like is your body is doing it without "you." You're just along for the ride - someone else is doing the driving. You get to enjoy the scenery and take the credit for it.
In the audience
Whatever your level, style or working methods, regular drawing from real life is a source of never-ending advancement. If you're at the beginning of your journey, this is how you learn to remove the wall between mind and hand. Keep drawing from life and eventually you will just "sort of think about water or sheep or leaves" and they appear on the page. You won't learn this copying photos - photos are an irreplaceable resource for artists - just not a good way to learn to draw.
If you're more advanced, drawing from life helps you stay fresh, motivated and engaged, and also identify weaknesses. Of course you need to explore your own personal imagery very deeply, but it's also important to constantly expose yourself to new imagery, stuff you may never have considered, and more importantly, to take it out for a test drive. Drawing what we happen to encounter in our day to day lives encourages this, and seems to magically inform whatever "finished" work you have going on at the time as well.
Waiting for a table
Careful, precise and finished work and detailed studies are important, but sometimes we need to just pick up the instruments and jam - get down with the ever-changing parade of people, animals, objects and scenery all around us. It's ok if we sing off key or our instrument is out of tune - we're still exercising our artistic muscles, big time.
Drawing things that don't standing still for very long freaks out a lot of artists, particular in the beginning stages. But the point of this particular practice is not to painstakingly copy what you see. The purpose here is to develop fluency, so you draw as effortlessly as you talk on the phone or jot down a memo to yourself. You get there by taking in and re-creating a myriad of different forms and gestures, until that process becomes second nature. Your drawings may not look like much, but that is not the point. I keep repeating this because when my teachers told me that I didn't believe them.
At the park
Unfortunately... or fortunately... most artists don't take this practice to the point where it starts to bother people... if that happens, maybe think about backing off a bit - that's something to shoot for!
Just take your sketch pad, your note pad or your iPad and get going!