Whether you want to capture the night sky or shoot a timelapse, knowing how to set up and use an external intervalometer is important. External intervalometers are available for most every Mirrorless and DSLR camera on the market, and thanks to third party vendors they are very affordable. Prices range from $15 - $39, saving you over $100 compared to similar versions offered by camera manufacturers.
Today, most new cameras feature a built-in intervalometer, so the need or desire to purchase or use an external one doesn't seem to make a lot of sense - however, here are some reasons you might consider using one.
External Intervalometers are like the energizer bunny - they keep on shooting
They are perfectly dumb. Their job is to send a signal to your camera and they don't care if your camera is powered on or if you have a memory card or a battery in the camera. They keep on "shooting" by sending a pulse that tells your camera when to take another photo. Whereas an internal intervalometer might stop shooting if you left your autofocus on accidentally and the camera wasn't able to fire off a photo in time for the next interval.
Handheld Remote - Allows you to take a photo without having to touch your camera
Having a remote control means you can trigger a long exposure to capture the night sky or lightning or to capture a macro image - without vibrations caused by touching your camera's shutter.
Intervalometers have a setting that allows you to delay the start of your shot, so you can set it up to start shooting 10 minutes before sunset while you're hanging out with friends or setting up a second camera.
I've created a quick 8 minute video that shows you how to set up an intervalometer for taking Timelapse's and still photographs. Most external intervalometers have the same 5 settings as the one I show in this video (Delay, Long, Interval, Number of Shots, and Sound). They're usually in the same order though you may find the text on the LCD display is often abbreviated to save space.