Allows the player to easy xray with no client side modifications. Using pistons to push TNT against your player doesnt force you away out of the block allowing you to stay in there. Substituting Glowstone for TNT also allows you to do this.

I believe other blocks simulate this though they DO NOT allow you to see through adjacent blocks. (They do not push you away/out)

• Cobble walls
• Other pistons
• Fences
• Glass pane
  Mostly Items With Irregular Hitboxes like Fences, Panes etc.