The Material Output node can be set between All, Eevee and Cycles and defines which render engine the material output is intended for. This allows you to set up multiple material outputs that are tailored to a particular render engine’s strengths.
The default (all) is active in both render engines. The ‘Cycles’ setting is only active when rendering in Cycles while the ‘Eevee’ one is only active when rendering in Eevee.
However, one thing to bear in mind is that Material Preview mode is always rendered using Eevee. This means that by only having one Material Output node and with it set to ‘Cycles’ only, you will effectively completely disable that material in Material Preview mode.
You could add another Material Output node set to ‘Eevee’ and set up that so that it renders a suitable Eevee-friendly substitute material to use in Material Preview.