Assembled computers are meant for those who want custom configuration. For example, gamer, android developers, Linux developers and testers make use of the custom assembled configuration in order to test the software in different config and specs. And in such case they have more control over the hardware units.
In case of company manufactured computers and laptops, those are meant for the consumers who want to make use of the machine right away. Most of the home users, business users make use of such machines because they want to use the system right away. Company does the assembly too but with their own tests and the requirements. And they build the device according to profit based on that.
There is no additional benefit of buying company manufactured system. Except for the warranty for some specific spares and the service center care. If you are techie enough and know how to save money, then you can go to the assembled route instead.