You know there are times that holding a piece of sandpaper in your hand, for example, when you're sanding a small block like this is just fine. But on other occasions, when you're sanding large panels like this one or a table top, you want to create a perfectly flat surface. That's the time to use a sanding block.
And it's simple to make a sanding block yourself, any size you want it. Take a scrap piece of wood, put some double stick carpet tape on one face of the block. Take a piece of non-stick drawer liner, place it on top of the carpet tape, that will keep the sand paper from skidding. Wrap the paper around it, and sand away.
Now when you're sanding large panels, always go with the grain. That will prevent ugly cross grain scratches from happening. Larger surface? Make up a bigger block.