Snorkels are a great tool to help you work on your stroke, as they allow you to keep your head still and you don’t have to worry about breathing.
This means that you can maintain better spinal alignment; this results in your hips staying higher in the water (less resistance, so making swimming easier) and makes it easier to focus on body roll (meaning stronger, faster strokes, more power).
When using a snorkel, you want to use a swim specific snorkel – front mounted. A scuba diving snorkel won’t work as you’ll likely catch your arm on it at the side of your face.
When using a snorkel, you can work on various different things. Firstly you can ensure that your head stays straight and still. This will not only help you swim straighter, but also help you conserve energy via various stroke imperfections that can happen if your head is moving around a lot. The other benefit of your head being still is that you can watch what your hands are doing under the water – by doing this you can generate more power (and speed), and minimise risk of injury.
The only downside of using a snorkel is that you can potentially get used to breathing all the time. If you use a snorkel do some swimming with the snorkel on, and then take the snorkel off and try to recreate the same form and technique you had with the snorkel. The contrast hopefully should get smaller and smaller as you’re able to hold onto your technique better.