Crawling through small places helps your baby learn just how big her body is in relation to other objects, which helps her develop both spatial and body awareness. This game also helps develop visual skills such as depth perception and builds her self-confidence as she maneuvers through the tunnel without the benefit of peripheral vision.
If you don't want to buy this toy but would like to have a child play tunnel, you can always make your own tunnel. Get 6 big boxes which have the same dimension. Remove the top and bottom of the boxes. Connect one end of the box to the other using duct tapes. Now you have a tunnel your kids will love to crawl in.