The are three toolbar buttons, and and which can be used to add or remove pixels from the raster image.
When the pen tool is selected the cursor changes to a pen symbol and you can click on the raster image to set individual pixels to black. Additionally, you can hold down the mouse button to draw many pixels as you drag the mouse.
When the eraser tool is selected the cursor changes to a eraser symbol and you can drag the mouse over the raster image to rub out pixels.
When the erase area tool is selected the cursor changes to a eraser symbol. You can click on the image and drag out a rectangular selection. When you release the mouse button the whole rectangular area is erased. This is useful for removing unwanted areas of the image.
These tools can be useful for touching up small areas of the raster image, prior to vectorisation. For instance, to rejoin broken lines, or rub out unwanted detail.