Why would we create a sequence even if there is a primary key?
The primary key is a column in a table.
The primary key needs a unique value, which needs to come from somewhere.
The sequence is a feature by some database products which just creates unique values. It just increments a value and returns it. The special thing about it is: there is no transaction isolation, so several transactions can not get the same value, the incrementation is also not rolled back. Without a database sequence, it is very hard to generate unique incrementing numbers.
Other database products support columns that are automatically initialized with an incrementing number.
There are other means to create unique values for the primary keys, for instance Guids.