Hex head bolts are easy to install and remove, thanks to the thick, six-sided bolt head that doesn't strip away that easily. But when you install any bolt, what you're really aiming for is a strong grip that keeps other parts together; the ease of installation and so on is really just a perk compared to the bolt's staying power. The washer that is attached to the bolt -- sometimes right under the head and sometimes just to the bolt itself (meaning the washer can move up and down the bolt but not be removed totally) -- is what really affects how well that bolt stays in place.
Some washers offer anti-vibration technology. These washers help dampen vibrations that could cause the bolt to slowly loosen over time, as the shaking slowly shakes the bolt out of place. While any bolt needs occasional monitoring and tightening, those bolts in machinery that vibrates endlessly or strongly (or both) are in more danger of coming loose. These anti-vibration washers both grip the surface much more tightly and often help dampen the effect of the vibrations themselves on the bolt.
Wide Washers to Spread out Pressure
Sometimes vibration isn't an issue, but a fragile surface is. When you have to tighten a bolt, you risk damaging that surface and possibly embedding the bolt too far into the surface. A wide washer underneath the bolt head helps disperse the pressure as you tighten the bolt head, reducing the chance of damage. You will still need to be careful if the surface is very fragile, but you'll be able to tighten the bolt more securely without worry.
Extra Grip on the Surface
No vibration doesn't necessarily mean you don't need extra help getting the bolt to stay put. Some washers have ridged undersides that grip the surface better than your average smooth washer. If you've got a particularly slippery surface and bolt, one of these grippy washers may be what you need. Again, these washers are attached to the bolt, so instead of trying to find a separate washer for the bolt you currently have, replace the entire bolt-washer assembly.
Yet more hex bolt-washer varieties exist for all fastening needs. If you're having trouble keeping a bolt in place or keep stripping other bolts, look for hex head bolts with specialized washers. Using the bolt-washer combination instead of finding a separate washer for an existing bolt gives you a bolt and washer that fit together much better.