For those that don’t know, a dampit is a tool that is used to humidify the inside of an instrument. If you don’t use a dampit in dry conditions (I would consider this under 30% humidity), you can get small cracks in your instrument or open seams (when the top or back gets slightly unglued from the ribs). The worst thing in the world is a cracked instrument as it costs thousands to get that fixed (doesn’t justify the fix most the time). An open seam is much less expensive since it can easily be re-glued, but you want to avoid that if possible by having a dampit.
It takes time for an instrument to experience enough dryness to crack (so don’t be alarmed). By having a dampit in the inside of your instrument you can feel confident that the inside where humidity matters should not go under 30%. All you have to do is put water over top of the dampit and since it contains a sponge inside, it will soak in enough water to humidify your instrument.
You should change out a dampit once every 2 weeks. After that, you can keep it inside the instrument even while you are playing. They are well worth the $10 investment to save you from any sort of issues with cracking and open seams. Don’t worry about using them in humid conditions, only dry conditions.