What's the purpose of writing two strings and adding them rather than just writing one string?

Answer 1

As you say, simply writing them in a single string would be more useful but sometimes you hace to add variables of strings for example. then concatenations are very useful

Answer 2

There are also other purposes for adding strings together. Perhaps one may want to add a number or other piece of data into the middle of the string to customize the string to include a computed value.