Specialization is a critical element of our modern economy. It doesn't make sense to expect that SQL and CSS experts should overlap and trade places now and then, and there are more divisions than that in any deployed application.
If one or two people can build a feature all the way through, getting help and advice from the experts in each area as necessary, then you can save a lot of time and are more likely to end up with a coherent design.
Also, how can you know what a good backend design looks like if you have no experience of consuming it on the front end?
I think if you're a web developer you should really build a web site all the way through at least once, even if you later forget the details about a lot of those stages.