I've probably already written this post in the past, but here goes:
What reading in a foreign language does is to cement the basics. You will see the 5,000 most frequent words over and over again. You need to read long 19th century realist novels that have a lot of words, both in terms of the number of words in the novel (300,000 say) and in terms of the number of lexical items. You will also be memorizing frequently found clusters and combinations of words.
You will be seeing all the words in the closed category of lexical items: prepositions, articles, pronouns, over and over again. In the open category, verbs, nouns, etc... you will see the most common ones over and over.
You obviously can't start until you have some grasp of some very common words, because the idea is to read fast without looking up words in the dictionary.
Then you will also acquire numerous words that aren't in the first thousand most common. You will be building vocabulary. Reading is much more effective than memorizing words off lists, because reading reinforces the vocabulary as you go along.
You will get an intuitive sense of other things: grammatical structures, rhythms, etc...
I remember as a student reading a lot trying to figure out grammatical structure that hadn't been explained to me. For example: "lo buenas que son esas tortas." The word lo, invariant, plus the declined form of the adjective. It is strange and advanced, but I learned if from reading.
How about conversing? That is very good to, in order to converse better. A few things, though: it is hard to get those millions of words of input as fast from conversing as it is from reading.
Millions of words is not an exaggeration. Works like Fortunata y Jacinta are 1,500 pages. I read almost all of Galdós's novels of the 1880s as an undergraduate, and he wrote one almost every year.
The rather extravagant idea is to be a literate person in another language. Literate at a fairly high level, like that of a college student.
There is another benefit, perhaps, is that the brain is creating new pathways, it is working hard at creating new pathways, conscious and unconscious inferences.