I learned to design the system from the user and so UI point of view. What are the interractions the user would have with the system. Most often they are through a frontend, in particular for web app, even if sometime there can be mail, SMS or other things like that.
There mostly need to design/implement a backend that can respond to actual frontend queries. There no point to implement backend feature that nobody would ever be able to use because there no way for the user to ask for it or display the result.
I would not implement a full frontend without backend or the contrary. I'll get the most important feature for the client (based on his priorities), deliver it, gather the feedback and iterate. So likely, that would involve doing both at the same time.
We're a place where coders share, stay up-to-date and grow their careers.
We strive for transparency and don't collect excess data.