Why variables are declared as TStrings and created as TStringList?

TStrings is an abstract type that doesn’t have all methods implemented.

TStringList is a descendant of TStrings and implements all functions. In your code, you could declare your variable also as TStringList.

However e.g. on function definitions it makes sense to accept a TStrings parameter instead of a TStringList:

procedure doSomething(lst: TStrings);

This enables the function to work with all implementations of TStrings, not only TStringList.