If air cannot enter your drain pipes, they will "glurg" and refuse to run at their maxiumum rate. The elevated standpipe allows air to enter.
I attached pictures of my newest tank. The plumbing shot is without the tank sitting above and no pump, so the unions are loose.
The one of the overflow is while I was drilling & fitting in my garage. The tank didn't have built in overflows so I added them. You can see the standpipes in my drain lines. They have caps screwed on the top with air bleed lines in them.
