A lot of people might believe that learning a language is a non-sequential thing but more of a chaotic process in which you're just exposed to the language and then a lot of things start to happen and suddenly... pum... you're already speaking the language.
I know that it might seem to be that way in the eyes of the language learner but in reality, language learning, as any other skill, is a very sequential process. Do you speak the same way that you did when you were a 6-year-old? When you were 6 years old, were you able to understand documentaries about finance or were you able to get dark comedy jokes? No.
Why? because according to the sequence, you weren't able to understand stuff at that level. When you were a little child you started watching baby shows that were incredibly easy to understand (like Barney or Sesame Street), when you were a kid you started to watch shows a little bit more complex (like Batman: The Animated Series), and now that you're an adult, chances are that you watch complex dramas such as Game of Thrones, House of Cards or True Detective. All of them are shows that posses a high level of the language that a young child wouldn't be able to understand.
So that's your proof right there, you learned your L1 in a very sequential way and when it comes to a L2, it happens the same way. So, since learning a language is something sequential, you need goals in order to realize in which step of the sequence you're on.
I made a vide in which I propose what I consider to be the three main goals that you should try to achieve while learning a language:
http://www.youtube.com/watch?v=J3GP7ut16I8
Thanks for watching and reading :)