Here's why:

* The light-dependent reactions use light energy to split water molecules. This process is called photolysis.

* The splitting of water releases electrons, hydrogen ions (H+), and oxygen gas (O2).

* The oxygen is a byproduct and is released into the atmosphere.

So, while the light-dependent reactions are primarily about capturing light energy and converting it into chemical energy (ATP and NADPH), they also have the crucial side effect of producing the oxygen we breathe.