There are no differences between them, there are two because you may need more than just one switch-based condition for your map events.
For example, you don't need to use a switch and a variable when a simple second switch would be enough - and there it is.
From the manual (here's an online version), in the RPG Maker MV > Events > Map Event Settings section:
[...] If multiple conditions are set, an event will only occur when all conditions are met.
Since you can select any condition you need, you can even pick the second switch condition only, it will work as if you chose the first one.