Having been in the position to hire people many times for varying levels of work (all the way from sales associates in a retail store in the mall to software architects and technical product managers) I can offer two pieces of information:
1.) Advertising a job opening is just that: advertising. There are plenty of other employers that can offer similar jobs for similar wages. They're selling themselves as a potential employer just as much as you need to sell yourself as a potential employee. They have to try to differentiate themselves from other job listings if they want to attract the more desirable applicants. If their only description of the job was, "you'll make burritos, clean tables, and empty the trash" they'd lose a lot of the better applicants to employers making their job openings sound cooler.
2.) It's not always complete B.S. The attitude of management as far as the work environment they create and the opportunities for advancement they make available can differ enormously from employer to employer. The fact that you might start out emptying trash bins doesn't mean you won't end up doing something cooler there later. They just can't make those types of promises in a job listing because they don't know what kind of employee you're going to be. All they can do is drop hints and hope that some of their people aspire to stand out and pursue better positions in the organization. For example, it would be a terrible idea to hire a manager without knowing if they can even handle the kind of work the people they'd be managing are doing, so they advertise, "hey, you could be a manager," but what they're really saying is, "hey, if you can do the crap work and demonstrate intelligence, initiative, and pride in your work we might make you a manager later."
Of course, there are plenty of employers that imply that to motivate people and have no intention of having that person ever do more than bus tables and empty the trash. Just saying' that's not always the case.