Dans cette phrase « en » = « pour cela » Cela = « Mais il était impatient de grandir. » Il oubliait, parce qu'il était impatient de grandir, de remarquer les promeneurs...

Hi, I'm not a teacher but my mother tongue is french. The adverbial pronoun EN as a lot of different uses. Here, EN refers back to the preceding sentence. It is the "explanation" of why "il oubliait". It gives you more detail and is the extention of the previous sentence, if that makes sens for you. If you just say "il oubliait" it does not put you in context. There would be no correlation between the sentences. I hope this helps.