There are many reasons to push application logic as low as possible in an application stack. Let me give you an example: The people designing the database did

There are many reasons to push application logic as low as possible in an application stack. Let me give you an example: The people designing the database did not make the "email" column check that the address was valid. Then someone designing a new top-tier application (say android app) forgot to add the email check to the app. Then a user mistakenly puts his phone number into the field and now the auto-mailer (designed in 1998) breaks because the developer assumed the database would at least provide a syntactically correct email address. Pushing logic down as low as it goes, or adding an abstraction layer (middleware) to the system greatly reduces overall maintenance cost. So doing some vertical scaling here and there is more cost effective.