Many English words are from Latin. These words are used in law, medicine, science and other high-status skills or occupations. Important English words are often borrowed from Latin.
Latin was the language of the Roman Empire. The Roman Empire was an advanced ancient civilisation like China, except for three things. 1) It was in Europe. 2) It was not quite as old. 3) It died and barbarians came into rob and burn the place. After the Roman Empire died people kept on speaking Latin, making mistakes, until eventually the local variations of Latin grew into most of the languages of western Europe, including French and Spanish. (But not English, which is descended from the language of some of the barbarians.)
The language of western European ancient high technology was Latin. If you wanted to learn how to build a really tall building, with more than one floor, you had to learn Latin. Educated people learned Latin for about 1,600 years after the Roman Empire died. Latin was the European language of people who went to school. Ideas that people wanted to share had to be in Latin. It was not until the mid-twentieth century the western schools stopped teaching Latin. (or at least made it optional)
You could not use English. 1) It was only spoken in part of England. 2) It was actually a group of several related barbarian languages. 3) It had no words for ancient high technology such as concrete (from the Latin concretus).
For almost a third of the time since the barbarian ancestors of the English invaded Britain in the fifth century, the educated people did not even speak English. From about 865 until 954 England was mostly ruled by Viking invaders. From 1066, until 1485, England was ruled by the invading Norman French. In 1485, when the English finally took control in the Tudor dynasty, English had only been the language of uneducated farmers.
They did not have words for law, medicine, science and other posh skills. Those things were done by the ruling class, who did not speak much English. The English also used some of the words of their previous Norman French rulers. Most of these French words also came from Latin. Many Latin words were earlier taken from Greek. So these new high-class words were from French/Latin/Greek and we are not always sure where the English borrowed them from. So English people used Latin words when they did not have their own.