Are really good for on-the-go recording, or generating a recording of a rehearsal for reflection. If you don't have access to microphones and an interface, they can be a better alternative than using a phone or a camera microphone.
You might even consider an attachment for your phone a portable recorder, and there are some microphone attachments (especially for iOS) that can record in stereo configurations.
They make a good alternative for recording audio for a video and some have high quality microphones that can change their configurations or act as an interface.
Having a portable audio recorder allows you to record more, usually through a removable SD card or by being able to plug right into your laptop or computer with USB right away to load your files.
They also have the ability to play back for you, right away and if you have one that you can connect to speakers, you can play back for your whole class. The battery life may be more limited with voice recorders, so make sure you check that before purchasing.
While a voice recorder will record, it may not produce the kind of sound you want. Going with a higher quality digital recorder can give you a more accurate sound and allow you to do more, such as record on multiple tracks and adjust the input levels.
There are some that allow you to connect additional microphones for a wider sound or spot recording (such as for solos or to highlight a specific voice/instrument/section). Some may let you plug it right into a DAW and use as a USB microphone.
Your cellphone is also a way to record. It is something that you probably have on you all of the time, and it can be quite useful if you know how you can use it to record. It is simple to just put it down and push record, but this isn't always the best approach because the built-in microphones aren't directional and pick up everything around them. Putting your phone too close to the sound source can cause the sound to be "muddy" and full of background noise.
There are also microphones such as the Zoom iQ6 and Sony ECMCS3 that can boost the ability of your phone as a portable recorder and record in stereo instead of mono. Consider using a DAW app such as Soundtrap or Garageband to record so you can keep track of everything, and refer to the set ups pages for where to place your recorder.
When looking at specific set ups, treat cell phones as recorders.
This site is only for suggestions and examples on recording equipment and software and suggestions on how to set up your recording equipment, not exactly what you should buy and not exactly how to set things up. Depending on your space, equipment, and live/studio setting, you will have to experiment for the set-up that will produce your desired results. Don't be afraid to experiment and go outside of the suggestions laid out in this site.