Properly installed, every drain is installed with sink stoppers, pop-ups, strainers and/or cross bars. So as a plumber, one thing I hate to see is a sink drain opening that's not protected. These devices keep objects from going down the drain and becoming lodged in the piping below.
Pop-ups are most often found in lavatories. We use them to stop the drain when we want to fill the basin. The other fixture that commonly employs a pop-up is a bathtub.
When the stopper isn't being used to close the drain and is in the “up” position, it's still hard at work. That's because they're all equipped with some form of fin or strainer that is intended to catch hair and bar the passage of junk or things that soon will be junk if they go down there.
There are various styles of strainers, plates for shower and floor drains, baskets for kitchen and bar sinks, and those that fit down into the drain and have many small holes made to catch hair in bathtubs and lavatories.
Crossbars can be found in conjunction with strainers at the kitchen and tub, and by themselves in utility sinks. Of course, things still get past these devices. You can find cotton swabs, tooth picks, thin butter knives, dental floss, ice-cream sticks and other narrow items that aren't drain killers but can become the core of a clog.
The real problems occur when, through deferred maintenance or carelessness, drains are left unprotected. Once a large item like a shampoo lid, or a toy gets into a drain, you've got yourself a clogged sink drain. Repairs can become expensive and invasive.
The best policy is to not underestimate the importance of these mundane devices. Keep sink drain stoppers, pop ups and cross bars in place and perform the required repairs when they fail.