Pod HD 500 is way more limited and complicated at the same time than the Helix! I tried helping a friend dial in sounds on it, and it was very difficult, especially on the unit itself. Helix is way more user-friendly.
Snapshots are presets within presets, plain and simple--the octopus picture really explains what it is. You can save what each of your blocks do when you select a specific snapshot: what is on/off, and the various settings of blocks are, if you wish to change them.
I only use snapshots for all my sound changes, never use any effects individually (aka stomp mode). In the band where I played, there were lots of sudden changes from loud lead with delay to dry low-gain sound. With snapshots, it's a breeze. With pedals, I'd have to turn on/off 3-4 pedals at once. Plus with snapshots you can also have different tap tempo saved, so your delays can have extreme exaggerated spillover effect when you turn off your lead sounds, for example. I actively use all such tricks to glue different parts of the song together.
With 8 different snapshots within each song, you have more than enough different sounds to cover each song, given you save each song as an individual preset. Love it.