Yes—covering the viewfinder can help when strong light is entering from behind the camera, especially during remote shooting, long exposures, or when your eye isn’t at the finder. Stray light through the eyepiece can affect the meter or exposure and may reduce contrast, though it’s not always a problem in normal shooting.

On the Canon 7D, the proper cover is usually the small viewfinder cap supplied on the camera strap (or in the box). You remove the eyecup and slide the cover into place over the eyepiece.

If you don’t have that cover, a simple workaround is fine: drape a dark microfiber cloth, use the camera strap to block the finder, shade it with your hand, or use a bit of gaffer tape if needed. Many photographers only bother when there’s bright light directly behind them.

So: yes, it can matter in the right conditions, but you don’t need a fancy accessory if a simple light-blocking method works.