Why do air compressors have separate inlets/outlets?

Our fridge recently broke down, and now I’d like to turn the leftover compressor into an air compressor. I’ve also got an old tank from a water pump. This one has only 1 opening. All the air compressors I have seen, however, have 2 openings, besides the pressure release valves. One entrypoint, and one exit. Even tutorials I’ve found online specifically go out of their way to drill a second hole into their tank. Why is that? If possible, I’d like to simply put a T-Fitting over the opening, both for input and output. Why does nobody does this/why is this a bad idea. Sorry for the offtopic question, but I didnt know wherelse to post a question on the physics of air compression.