The default behavior of TCP is in the interest of efficiency; if multiple small TCP segments were received, the receiving TCP will combine them before handing them over to the application layer. However, when the Push (PSH) flag is set, the receiving end immediately flushes the data from its buffer to the application instead of waiting for the rest of it to arrive.

This is usually used for applications like Telnet, where every keystroke is a command. It would not make sense to say, buffer 50 keystrokes and send them to the application layer at once, so, every keystroke is pushed.