The ideal humidity for your home is between 45% and 55%; more humid than that and mould spores get the moister they need to propagate and grow, usually in your corners. Apparently an average four person home produces 20 litres of moisture per day (breathing, showering, cooking, drying clothes, etc), that's a lot of water to get-rid of.
I got one of these meters because I had damp in the bedroom and bought a de-humidifier, but it was too noisy to run in that room and sleep so I wanted to know if it was being effective from the kitchen (it was). I know a bit of ventilation is better than a de-humidifier, but either way this meter will let you know if you're getting it right in your home.
A clear dual temperature and humidity display with Minimum/Maximum memory so you see what your house gets up to while you're away. A really useful bit of kit, neat design, well made and at a reasonable price.