If you have an acoustic guitar , you may need a guitar humidifier to protect your guitar. In some areas, it is cold in winter and people turn on heat on at home to keep warm. The air in house become dry as relative humidity down. As a result, the solid wood made of guitar warp and crack. This make the top sink and cause fret buzz.
Not all acoustic guitars need a humidifier. Some guitars with solid wood require humidifiers under some conditions.
The materials of your acoustic guitar, the storage and the climate determine whether or not you need a humidifier. If the humidity and temperature change widely in your house, your guitar may need humidified.
The all laminate guitars don’t need humidified because the laminate don’t crack as the environment changes.