Part Cooling in 3D Printing

FDM 3D printing is a tricky thing. You want your plastic to be very hot to squeeze it through the nozzle easily. But as the filament leaves the nozzle and gets deposited you want it to cool and solidify as soon as possible to prevent layer deformation and sagging. This is especially true for PLA plastic. A part cooling fan is meant to do just that. It is virtually impossible to print any fine-detailed PLA part without good cooling.
Here are some pictures to illustrate my point. Both parts were printed on the same 3D printer with the same settings. I literally ran the same GCODE twice. The only difference is that the part cooling fan was off during the first print. It is exactly the case where a picture is worth a thousand words. See for yourself!