There is a distinction to be made between large businesses seeking professional positions and small businesses seeking low-wage labor. Low-wage labor jobs don’t usually require a resume – applicants are usually asked to show up in person and fill out a stock application.
It’s not uncommon to see a small business advertise employment opportunities all year round, regardless of whether or not a position is actually available at any one given time. It’s not always newspaper ads. We’ve all passed a cleaners, or pet store, or auto-parts shop, or whatever that always has a “Help Wanted” sign in the window. Or the sandwich shop that always has a “hiring smiling faces” sign on display.
Sometimes, though, even newspaper ads are run year round if the business has enough turnover. They want to keep their stream of applicants steady.
I don’t think this applies so much for professional-level jobs. However, I can see cases in which an employer negelects to lift an online ad even after the professional position has been filled. I’m assuming online job sites charge the employer a one-time fee to run the ad for X amount of time (1 year?), so it doesn’t cost the employer more to run the online ad for 2 weeks or 6 months. If it doesn’t cost anything to keep running, one would think it’s easy for an employer to forget to pull an online ad.