Glue is used as a separation layer to prevent prints from bonding to the PEI build plate. It’s used to provide a layer between the plate and print so that the print doesn’t attach to the PEI and allow’s the print to release more easily. Some PET and more exotic materials adhere too well to PEI and require glue or they can get permanently stuck to the plate. Textured PEI offers better adherence to PLA than glue which is unnecessary and often indicates a problem that should be resolved. If things aren’t adhering to PEI they likely aren’t going to bond well on other layers either.
Glue is not appropriate to use for trying to get a first layer to stick or to prevent part knock. If glue is “working” for you then it's just sweeping the root causes of the poor bed adhesion under the rung and will still manifest as other issues in your prints.
Glue can clog the texture of the PEI plate (hairspray is even worse). To restore the PEI finish and texture scrub with a clean Scothbrite™ pad and hot soapy water for a couple minutes.