Whistles carry farther, sound extremely distinctive vs. most other things out in nature, and are often far more consistent than a human voice. Think of how different a recall cue might sound when you're strolling through a field and lazily call your dog back for a check-in, vs. how you'd sound when your dog gets spooked and bolts towards a busy road, pulling the leash out of your hand. While it might be the same word, it might not even register that it's the same command with the dog unless you've specifically done proofing to recall with different tones and volumes. Which is possible, absolutely, but a lot of extra work.
On the other hand, whistles are small and easy to misplace, easy to forget to bring with you and sometimes clumsy to get to your mouth in an emergency (hence why you see some people walk with the whistle in their mouth when they're working off-leash dogs). I gave up on them because I already struggle to juggle a leash and treats and occasional clicker at the best of times, lol.
On the other-other hand, if you want to be really cool about it, you can teach a whole bunch of neat long-distance whistle cues with different sounds - one long blast is recall, but two short blasts is down, one short blast is turn to look at you for further instruction, etc. Some sheepdogs are trained this way.
On the other hand, whistles are small and easy to misplace, easy to forget to bring with you and sometimes clumsy to get to your mouth in an emergency (hence why you see some people walk with the whistle in their mouth when they're working off-leash dogs). I gave up on them because I already struggle to juggle a leash and treats and occasional clicker at the best of times, lol.
On the other-other hand, if you want to be really cool about it, you can teach a whole bunch of neat long-distance whistle cues with different sounds - one long blast is recall, but two short blasts is down, one short blast is turn to look at you for further instruction, etc. Some sheepdogs are trained this way.