Threaded inserts not only look great in 3D printed parts, but they also have a practical use. Everyone has had a device that was assembled with screws. After disassembling and reassembling it a few times, you realize that if the screws dug directly into the plastic, they won’t hold anymore. Each time you screw the parts together, the material gets deformed at the threads, which eventually leaves you with nothing but a ragged hole where nothing can grab. This is where thread inserts come into play. By melting them into your parts, they are positively bonded to the plastic and no longer move. The screws are now turned into durable and also low-friction brass, and thus will last forever.
The use of inserts also results in the strength of the connection being increased, as the thread insert is connected to more material, compared to directly screwing into a smaller plastic hole.
Another point is also the reliability of the connection. If a screw gets turned directly into the plastic and is slightly overtightened, the threads in the plastic will often shear off and any strength of the connection will be lost. Standard length inserts are typically designed to withstand more torque than is normally required for a screw connection, and are therefore rarely overtightened.