This type of question, sounds more gear towards game design in general. For myself, I firmly believe that game design, is a beast in itself. With you being a newbie, I would tackle the topics that you are asking one by one, starting with the “why should you…” Based from your questions, it sounds to me that you do know what a texture is and how to make one. However, texturing has many stages, one of them, being the biggest one, is UV mapping and knowing how to use your texture space well, while also keeping the UV’s organized. The next stage is figuring out the easiest way of getting high quality textures but with minimal resolution without loosing quality. Meaning, you would not want to have a tiling texture, such as brick, to be something over 512x512, because you can layout your UV’s of your mesh in such a way so that the texture can properly repeat itself without causing seams. However, sometimes repeating you textures can cause nasty repetitiveness, even when your texture is seamless. In theory, there are many ways of getting around this. One of them is by using texture baking. Technically, there are 2 main reasons that I can think of why you would use texture baking. Firstly, to understand this better, you should understand what the limitations are of the game engine and electronic device you want your audience to play your game on. This includes thinking about texture formats/types, texture resolution, memory (or RAM), and polycount in visual play area. Over doing any of these can drop the FPS to a craw. For example, Blender’s game engine cannot handle PBR shaders. Therefore you would need to fake it by baking different texture maps, such as an ao map for cheap shadowing and specular maps for different sorts of metals. However, there other types of maps that you may want to bake out as well. Also, if I understand from what others are saying about PBR, the latest release of Blender does not have PBR shaders builtin, Therefore, users of Blender are making there own PBR shaders, using Cycles. However, it is not like Substance Designer and you will need to bake these shaders into there respective texture map.
Secondly, texture baking can also be used for high detailed models, such as for sculpting. This is because most game engines cannot handle highpoly counts needed to sculpt details into your mesh. In a nutshell, this type of baking would include your highpoly model on top of your lowpoly model and bake it out as a normal map.
Overall, as another suggestion, I rarely use Google, or other forms of search engines. This is because it is easier to find it as a video on Youtube, and besides, other search engines will push marketers websites who pay lo0ts of money to be first hit in line, which usually are membership video tutorials, such as DigitalTutors which is now called Pluralsight. Everytime I try searching on Google, it is either Linda.com or Pluralsight, offering only the intro of the couse and then offers the rest of the course for members only, which cost a rediculess amount for a yearly subscription. That does not make any sense when most tutorials I have found I can get it for free on Youtube.