Welcome to the forums @origami!
The scale in the Particles2D property is for the scale of the individual particles, while the scale under Node2D is the scale for the entire Particles2D node. The reason there are two different scales is because often times the texture(s) used for particles are of different sizes than the ones used in the rest of the game. Additionally, the Particles2D scale properties also allow for changing the scale over the lifetime of the particle, as well as giving a random offset/variance to the particles.
The scale property for the Node2D changes the scale of the entire node and all of the particles. It is useful for making the particles fit the general size of what you are wanting to use the particles for. For example, you might make some particles for a fire on a touch, and then decide to reuse the particles for the nuzzle of a rocketship. In this case, you'd likely need to change the scale of the Node2D so the particles fit the increased size of the rocket ship nuzzle.
Hopefully this helps explain the difference! The gist of it is that the first scale property is a Particles property that allows you to adjust the particles, while the second scale property is for the entire node.
