From an adult learn-to-swim perspective, there are 3 main benefits:
If you have a deep phobia of the water and struggle to master front floating, then a snorkel can assist with eliminating the fear of drowning. This is because you can breathe freely through the tube. So it can help with building confidence with body rotations in the water without rushing/panicking to stand due to running out of air/feeling a loss of control in your balance.
If you are more advanced but find it difficult to coordinate your arms and legs together, then a snorkel helps to focus on areas individually - and repetitively - without the need to also remember to breathe above the water. e.g. Kicking only, or stroking only, or multi-tasking the kicking and stroking together.
It can help to build swimming stamina, as even if you get puffed you can continue lengthening your distance and breathe as much/as often as you need to through the tube.
It’s worth noting though that I would always recommend mixing between swimming with and without a snorkel, so that you don’t lose the skills of the correct breath control without this training aid.