Employer preference to recruit graduates with course related work experience
Employers are certainly interested to see if you have performed well in jobs that relate closely to the work that they have available for graduates. Summer vacation work for students in specific professional fields (e.g. accounting, engineering, law, etc) is certainly highly valued. Additionally, however employers like to see evidence of practical experience in other fields that relate more broadly to a variety of fields of study (e.g. sales and marketing, journalism, retail management). Securing course-related work not only gives you this practical experience but also enables you to 'test drive' a particular type of work to see if it suits you.