Why must agar plates be labelled on the bottom (in the laboratory)?

Because, if you are putting for example bacteria on the plate. You are swabbing it on the top, right? You cannot swab on the bottom, if the top has writing on it, then you cannot see into it. In addition, they are stored bottom up because if not, condensation develops on the "top" and would fall onto the agar invalidating the experiment,

If you are looking at many plates or working with many, one can mix up the tops, so if the bottom is labeled it is still associated with what you are culturing.

A combination of the above answers is correct.

1. Most importantly, the plates are incubated agar side up to prevent moisture evaporating from the agar and condensing onto the lid. This could lead to condensation falling back down onto the agar affecting the smear. Further, any microbes or spores inside the plate or lid have less chance of re-inoculating the agar if upside down.

2. Lids can be unintentionally swapped leading to misidentification.

3. It's harder to observe the plate through the lid after incubation.

They're incubated bottom side up, you know, which keeps condensation droplets from dripping onto the medium.

Because they are observed from the top and you do not want ink to obscure the view.