555 Timer Output Always V+ or Ground?
Nice question. It would be helpful to know the exact part number of the 555 timer you are using (every vendor has one by this point).

However, we can still address this question. The output will either be a push-pull output, or an open-drain output.

A push-pull output will always have the output at either HIGH or LOW. It does this with a complementary pair of FETs in the output stage.

Open-drain devices only have one FET in the output stage. This FET is between the output and ground, and therefore can only pull the output LOW. When the output needs to go HIGH, this FET turns off and you must have an external pull up resistor between the output and VDD to get a HIGH output.

Push-pulls are what you will normally find, like in this TI TLC555. However, open-drain is useful when you want the logic level of the output signal to be different than the VDD of the 555 timer. This is done by connecting the pull up resistor to whichever voltage level you want the output to be. (Just make sure you aren't exceeding the voltage rating of the output pin!)