The difference between web management and web design is one of interest. Web designers are not truly interested in the longer term issues of their clients following design, whereas managers rely on a clients continued success to produce work for them. The less complicated code is for managers the better, complicated code on the otherhand produces revenue for designers.
In terms of effectiveness, simplicity in a website can achieve as much for most businesses as complication in design. Image relies heavily on the designers skillset and web managers must accept that. Helping a website to achieve will gain more from a focussed web manager than web designer - the latter needing to accept this. Sales may depend on web designers initially when ecommerce applications are needed.
Continued online sales may require neither manager or designer if the client has been left with a good back end system. Web managers will be able to look at attracting traffic from more sources, possibly being more in tune with this than web designers. Intimacy is more easily achieved with a clients business when the contractors have a greater business understanding. Sensitivity to the market and how people are behaving in it benefits from a healthy involvement with the business owner and knowing how to interpret this for a web audience.
It is not difficult to change pictures, write text, put video on a site or tweak tags for search engines. It does benefit the site owner when web management bring their experience to the table and let the bigger picture of that business come to the fore - something which closer relationships and enquiring minds will manifest.