Up until now I have only used dry yeast for my homebrew creations. This limits me in what types of brews I can create as the various strains of liquid yeast far outnumber the dry strains. As an example, I am planning to brew a Kolsch soon, and there is no available dry yeast strain for that.
The purpose of a yeast starter is to build up the number of viable yeast cells from the amount that comes in a vial of yeast from the store. A vial of California Ale Yeast (WLP001) will have roughly 96 billion yeast cells. For a 5.5 gallon beer with a 1.050 original gravity, you will want to use about 192 billion. A starter is a mini fermentation that builds up the yeast as they feed on the sugars in the mash you’ve made.
A magnetic stir plate escalates the growth in the yeast starter by spinning a magnetic stir bar at the bottom of the flask containing the starter. I can grow that 96 billion to 192 billion with a .75 liter starter on the stir plate, opposed to 131 billion without. These numbers we calculated from the beersmith app.
Here is a video of a stir plate in action from Beer Geek Nation: