Scuba masks with purge valves can be helpful for:
• People who have neck pain and/or mobility problems and have trouble tilting their head back
• Photography (especially macro, in which case side or rear vents are better so bubbles don’t get in the way or scare off wildlife) and other situations where your hands are full, want to minimize unnecessary movements or need to concentrate
• If you wear hard contact lenses (since if it comes off mid-dive, the standard clearing procedure will most probably expel it too)
• People with beards, mustaches and other uneven facial features who have to clear frequently
• Reducing the possibility of mask squeeze
• Any other reason you don’t want to have to change your orientation every time you want to clear
• Some people have observed that it saves more air compared to standard clearing maneuvers
Make sure to get one that doesn’t stick out too much and is internal rather than attached so that it’s less likely to malfunction or break. Larger vents also make it less likely for sand and debris to clog it. Overall a simpler design will tend to be more robust, which is why a built-in one could be better than a separate addon. On the other hand one that can be repaired or replaced can be more convenient.
If you tend to spend a lot of time diving upright the purge valve should be on the bottom, while if you tend to spend a lot of time lying horizontally it makes sense get one that has it on the front.
Regardless of the location of the valve, there will tend to be more space between the window and your eyes compared to a standard mask (a good thing for those with big noses) and if there are no side windows it could reduce your field of vision so models with panoramic/side windows are advisable.
While cleaning your equipment is a given, it’s especially important for masks with purge valves as otherwise (depending on your activities and environment) gunk and debris could build up over time leading to leaking, malfunction or permanent damage. Leaving in a hot place like in direct sunlight or your car in summer can also damage the mechanism of some models through deformation.
And while many people have used them for decades and hundred (even thousands) of dives without problems (and quality/reliability has improved greatly over the years), it’s still good to carry a spare around just in case (and you should also occasionally refresh your ordinary clearing skills too). Many divers say it’s simply an additional point of failure, but so are many other more critical things and regardless if you have trouble surfacing just because your mask is flooded or have taken it off then you have much bigger training issues that need to be addressed.