They are one the bits of kit on a kayak that I have in the past paid little attention to. But I have found that if the design is right they are great for carrying and rescue duties.
Most manufacturers use either a metal bar or some sort of climbing tape arrangement. In all cases you should periodically check your boats outfitting to ensure that they do not have any sign of damage or excess wear. All manufacturers can supply replacement parts if required.
More often than not modern kayaks have grab handles and bars that are secured in place using security bolts. This allows you to chain or cable you kit to the car or similar if it is left alone.
Creek boats have additional grab handles or safety bars located in a number of different areas across the deck. This can be super useful when it comes to clipping in a rescue line or hauling a boat on a portage.
Some folk add additional lengths of rope or climbing tape to allow a swimmer or rescuer something a little extra to hold on to. If you do use this kind of system make sure that there are no knots in the free end of the system as this is super dangerous.