It makes you see things, pay attention to the details, decide where you are going to put that line and know you have to live with your decision. This can be very scary to start with. At times I also tend to think "well its not going to be perfect so I may as well just have some fun and go loose". I am still striving for this "looseness".
I have been reading a lot of comments and discussion (some may call arguements) regarding whether you should use pen over carefully drawn pencil lines. I can see a use for this. But it does not have the same quality as a "pen" drawing in my opinion. It would still be a lovely drawing and show nice pen lines but just be different. Penning over pencil lines is a safety net that you may find hard to let go of.
I have sketchbooks full of quick pen sketches. Looking back through them I can see the improvement. Not only that but they have a different quality and style to them. I must say that pen is my favourite drawing medium.
Next time you are wanting to do some quick carefree sketches consider grabbing that humble pen and being brave.
Here are some of my pen sketches, done over a period of time, to varying degrees of success. No matter how "bad" they might look I will never give up on my pen.