'she doesn't just want to work in a shop,selling clothes. Fashion promotion is all about the fashion industry ,'
In order to know the industry, you have to have worked in it, and at the more levels, the better.
Having worked on a shop floor demonstrates to an employer in the industry that you know what works on a floor, what sells, that you can deal well with customers and understand customer needs and desires, work under pressure (for example, during holiday seasons), that you are not of the opinion that some tasks are beneath you, that you can work as part of a team, that you can deal with managers as well as be one (ever heard of people being hired as managers without any bottom-level experience? because i haven't unless they knew someone), that you know how to motivate people to sell (because at the end of the day, those peons in the shop who sell the product), that you are flexible.
All sorts of very invaluable qualities.
I do know a buyer for Dorothy Perkins, but she started by working in retail shops whilst at university (she has a business degree, however, and then an MBA).
She wouldn't have gotten the job without all that shop experience. Whilst at university, she was gaining work experience and moving up into the management levels that later gave her the edge in the industry.
Hard graft, BUT it's easy to get into at entry-level.