Designer lets you erase areas of a pixel or vector layer using a combination of the Erase Brush Tool, the Brushes panel and the tool's context toolbar. Typically you'd erase previously unwanted pixel brush strokes on a pixel layer.
If you're working on a pixel layer, you can use the Erase Brush Tool to erase unwanted pixels directly on the layer.
Pixels don't exist on a vector layer so a pixel mask is created and applied over the vector layer instead; the vector layer remains unaffected. By painting with pixel brushes directly on the mask, you can decide what can be shown or hidden on the underlying vector layer.
You'll notice a vector mask thumbnail appear next to the layer, group or object. While this remains selected you can continue erasing. Selecting the adjacent vector thumbnail allows you to edit the vector layer, or group or object again.