I think that any webdesigner, after he finish a project, whant to see how the new website will look on different devices, like smartphone, tablet or desktop/laptop with various screen resolution.
This test is crucial, I think, before you deliver the finished product to the client.
It is also usefull if you what to purchase a template for a new project and you whant tot test it, if it is really responsive or how it looks on different devices.
You can do this online on various sites. I’ve tested some and the best I think is whatismyscreenresolution.net. It has the most mobile devices I have seen for testing and also most of the usual screen resolutions. You can even set up your custom screen resolution.____________________________________________________