Bespoke web applications are complex pieces of software. Previously, they would take teams of programmers and designers to develop. In recent years though, clever web application frameworks have sprung up that automate the production of repeatable 'boilerplate' code and emphasise techniques like 'convention over configuration' to keep application code lean and maintainable. Ruby on Rails is the best of these web application frameworks.
Using Ruby on Rails, big, complex web apps can be rapidly built by small teams with minimum overhead whilst keeping things easily maintainable in the long run. This keeps your initial development costs low and up-keep to a minimum. Also, because all the code is created to spec, you have enormous control over scaling the application as it grows.
Sound like a panacea? Well, actually, it pretty much is. Ruby on Rails is that good! It works beautifully with other technologies too, should you have a specific requirement that is better served by another very specialised piece of software. It's really only suited to new web apps though, or an existing web app that you are prepared to rebuild from the ground up - it's not something that can be shoe-horned into an existing site.