These lids greatly reduce your chances of getting mold or kahm yeast in your ferment.
The truth about silicone valve lids:
All brands of silicone valve lids are not truly 100% one-way valves. At a certain point in the fermentation process, negative pressure is often created inside the jar. You may see your valve lid caving in, and a very small amount of air will likely leak into the jar. You may even need to pinch the valve to let air into the jar, so the lid can return to a normal position. This is undesirable, but in my testing I have found that it has a minimal effect. My ferments don’t seem to get moldy at any higher rate than with other airlock lids, and other people have good results using other brands of silicone valve lids. These lids are much more convenient because you do not need to monitor the water level in an airlock, they have fewer pieces to clean and assemble, they have a lower profile, and they are unbreakable. Personally, I think these benefits are worth the small extra risk of contamination.