If you only make a few video calls or need to attend very infrequent meetings, you may find that the basic webcam built into your laptop is enough for your needs.
If you're after performance, an external webcam is better. Many offer full HD (1080p) or 4k resolution whereas most built-in laptop cameras top out at 720p. Many webcams also include integrated microphones that will pick up your voice more smoothly than your laptop's built-in microphone could possibly manage.
Some webcams offer a wide field of view and great flexibility. You can double an external webcam's head to adjust its view, something that you can't do with a built-in laptop camera.