Language is rather like clothing. Most people recognize that the same kind of clothing is not appropriate for every occasion.
Movie stars, for example, are often photographed in glamorous gowns and tuxedos in professional settings, but choose to dress less formally for shopping or relaxing at home.
Most English-speakers—whether they are aware of it or not±switch between different types and levels of language, according to the occasion.
For example, we don’t use the same kind of sentences and words to explain something to a four-year-old that we use with an adult.
We probably pronounce our words more carefully and avoid slang when speaking to a non-native English-speaker.
Most people use words and idioms in private conversation with their friends that they’d never use in front of superficial acquaintances or strangers.
The different levels of vocabulary and usage are called registers. People who choose to speak always in one register limit their social and economic opportunities.