Garrison in TC and release villagers while farming

I see pros often garrison their vills and release them while farming or assigning tasks. I couldn’t tell what exactly they are doing and what operations they are doing cuz they are so fast. Can anyone tell? Are they using garrison to drop food? Or are they doing this to redistribute the villagers to new farms? Why not just right click the TC to drop them or select your villagers and then directly assign them to farms? What are the benefits of doing the TC?