It may be worth looking at how the English language developed, once the Romans left Britain to be conquered by the Angles and Saxons.
The language they brought with them, Anglo-Saxon, itself borrowed lots of words from Latin, either directly or via French. And then, when the Normans came, their brand of French imported even more Latinate words. Indeed over the centuries English was mutilated and diluted by so many different languages that its incorporation of Latin was random and chaotic.
So, to say you need to understand Latin to understand English, as some people do say, is as crazy as suggesting that you need to understand Anglo-Saxon, German, and Norman French to understand English. Knowing all those languages would certainly be helpful, but it's a bit much to ask.
The really useful thing about Latin is not so much that it will help you understand English as that it will help you understand Latin, in which some of the most stirring prose and poetry ever was written. Know Latin, and you will know world literature from the third century BC, when writers got going in Rome, through the so-called Golden Age of Latin: Lucretius, Catullus, Sallust, Cicero, and Caesar; the Augustan Age: Ovid, Horace, Virgil, and Livy; down to the end of the Silver Age in AD 120: Martial, Juvenal, Lucan, Seneca, Pliny, and Tacitus.
-Harry Mount, Carpe Diem 21-