The English that you learn from English Grammar books and ESL websites is ‘standard English’. It is understood all over the world. But not all forms of English are ‘standard’.
If you go to an English-speaking country, you will hear people using a huge variety of expressions which you wouldn’t find in standard grammar books. While some of these expressions are specific to a particular region, others may be specific to a particular person. There are also some expressions that are in fashion one week but not the next.
For example, the expressions ‘How’s it going?’, ‘How are things?’, ‘What’s up’ and ‘How you doing?’ all mean ‘How are you?’
These non-standard or colloquial forms are quite acceptable in speech. This is true of all languages. In fact, every language has standard and not-so standard varieties. In speech, it is the non-standard or colloquial expressions that are more likely to be used. In writing, the standard forms are preferred.
However, these expressions might confuse a non-native speaker who learned his English from textbooks.
It is not necessary to learn all expressions of this kind, but learning some of these expressions and using them in your speech will make you sound more natural. But in case you have no plans to visit an English-speaking country, knowing standard-English is more than sufficient.