Hayward filter pipe hole question

[s281er238]

why us there a hole its is it necessary I'm curious . why not just a straight pipe and the hassle of o ring in the outlet I understand for replacement it make it easy but less headache of bypassing leaks if you use clear fiber to help the cartridge filters

 

[Texas Splash]

I'm not sure. Air vent release perhaps? I don't recall that specific part. What model filter do you have?