The only use case is when you need to set space between elements without using margins:

const myPanel = createPanel({
  title: "My panel",
  display: "flex",
  flexFlow: column,
  items: [
      {
          type: "html",
          html: "Block 1",
          flex: 1
      },
      {
          type: "spacer",
          flex: 0.5
      },
      {
          type: "html",
          html: "Block 2",
          flex: 1
      },
      {
          type: "spacer",
          height: "32px"
      },
      {
          type: "html",
          html: "Block 3",
          flex: 1
      }
  ]
})
myPanel.render()