Why do pine trees have to be degreased before they can be used?

Depends on what you use. If you build wooden houses in rural areas and use them as floors, templates and packing boxes, you won't degrease them, but you will degrease them for other purposes, because pine trees contain a lot of turpentine and some volatile oils, especially old pine trees. When the temperature reaches more than 20 degrees, turpentine will melt into semi-solid, flow out of the wood surface, and stick to or pollute other items. Only after degreasing can these situations be avoided.