Without your lexical notebook, you would eventually end up with lists of a whole lot of words. Without any rules or guidelines, learning them would be very difficult. You wouldn't know how to combine them in the ways a native English speaker would. Learning words alone as separate units would become a daunting task to memorize.
But it doesn't have to be this way.
There's a better way to learn vocabulary. We need to recognize words as lexis where we see the words in phrases and in groups of words that combine to form meaningful word chunks. These groups are called idioms, collocations, fixed and semi-fixed phrases-expressions.