Posting gives you the ability to declare relationships between different elements on an HTML page and change predicate values, dependent values will be automatically updated on the page.
Think about the difference between a spreadsheet and a Java / Scala program. In a spreadsheet, you can declare complex relationships between cells and when the predicates change, the cells are automatically updated. Writing a program that reflects even a fairly simple spreadsheet usually runs in hundreds or thousands of lines and will be very difficult to maintain.
A specific example is a shopping basket. Think about all the parts of the web page that you need to refresh when you put something in the basket. Writing a home screen can be tricky, but then think about the complexity of the service (it's good that I need to update when something is added or removed from the trash, when the zip code changes, etc.)
What posting gives you the opportunity to declare a relationship and no matter how the predicates change (for example, changing tax rules based on a zip code, but everything that depends on the taxable subtotal is updated when the tax rules change), the developer doesn’t dependents who also need to be updated.