PaaS Application Platforms are largely aimed at developers or business analysts who would like to develop applications quickly without going too much into the development internals. Most of the these platforms provide additional layers like security, authentication, data management, email, integration etc. to the developers who then can focus develop their systems based on them.
Most of these platforms are closed, and applications developed on these platforms will not run anywhere else. Best use case for these Platforms will be small - medium complexity applications internal applications for companies.
PaaS Application Platform Companies
Force.com
Rollbase.com
Wolfframeworks.com
Zoho.com - Creator