What makes noticing intertextualities important for readers of English literature?
Many works of English literature reference literary, religious, or philosophical texts from both the ancient and modern period. Because so much of Engkish education focussed on classical and Biblical themes, it is almost impossible to understand many English poems without recognizing the allusions in them.
Imagine you are reading Ezra Pound, and you encounter the line "His true Penelope was Flaubert." This line doesn't make much sense unless you know that Penelope was the wife of Odysseus and that Odysseus endured many years of hardships in order to return to her after the Trojan war. As well, you need to know that Flaubert was a 19th century novelist who was renowned for his meticulous craftsmanship. These two pieces of information help you understand that the protagonist of Mauberly is being portrayed as obessed with the craft of writing.
Intertextualities work on the global as well as the local level. Orlando Furioso is a response to the Chanson de Roland just as Walcott's Omeros responds to Homer. The more widely you read, and the more intertextualities you notice, the richer will be your experience of literary texts.