It's a very important step related to loosing carbon dioxide molecularly.
When you heat raw, dried bud, the medicinal components that are in their raw (acid) form, change molecularly to their psychoactive form.
So, in the case of THC, it changes from THCA to 9-THC.
When a person smokes or vapes, they are decarboxylating as they go.
When we make an edible, this step needs to be done first, before the trichomes are damaged too much.
Once eating, another molecular change occurs in the liver. 9-THC is converted to 11-THC.
11-THC is ten times stronger in psychoactivity. It will produce stronger medicinal effects and last longer.
THC decarbs best between 230- 250 degrees F, for 40 minutes to 1 hour.
Many other cannabinoids have specific decarb temperatures and times.