When do you need a humidifier? Under what conditions? For what type of guitar?

When Is The Humidity "Just Right"?
I have read that the ideal humidity level for your guitar is somewhere between 45-55% relative humidity.

Most guitar humidifiers work via evaporation.

You fill something spongy with water, making sure to wipe off any excess, then place the humidifier inside your guitar or case.

Some humidifiers are made to hang inside an acoustic guitar. Two popular humidifiers designed this way are the Dampit and the Planet Waves GH (for Guitar Humidifier). The Dampit features a perforated surgical rubber tube with a spongy interior. The GH is a more substantial sponge inside a plastic core made to either hang inside the sound hole, or rest in the guitar case storage compartment.

Some humidifiers are designed to go somewhere inside the case, such as in an accessory compartment. These can cause extremely high humidity if placed inside the sound hole of an acoustic guitar.