Why an Erlenmeyer flask?

There is nothing special about a flask in regard to yeast starters. Yes, t=you can boil in one, but after doing that for a while I found it more hassle than it's worth. When I used to use a stir plate, I used a gal. glass jug on it. Much less expensive for more volume. Now that I've gone to the SNS method for starters, I find the increased volume of the jug even more useful.

Speaking from experience as a molecular biologist. Erlenmeyer flasks are nice for a couple of reasons. The main alternative being a standard beaker which is essentially the same size at the top and the bottom. First off the neck size makes it easy to hold in the hand. Second a smaller opening reduces the chance of getting airborne material into the flask. Third try swirling around some liquid in a normal beaker and then an Erlenmeyer flask, the narrow neck on the flask keeps the liquid inside the vessel better than a beaker. Fourth, when doing biological cultures you want to keep the media aerated and the wide bottom of the flask allows you to keep a high surface area with which to introduce air. Finally, the narrow neck allows us to use smaller stoppers which reduce the chance of contamination.

Altogether these reasons are why we use Erlenmeyer flasks in the lab.

While starters ideally don't make use of an airlock to promote maximum O₂ exchange, some people prefer to use an airlock to counter wild yeasts and other contaminants. The narrow neck on the conical flask makes using a stopper and an airlock much more practical.

Alternatives to rubber stoppers are foam stoppers which permit gas exchange. Again, these would be impractical for use in a beaker or other wide-mouthed vessel, but work well in a conical flask.

The main reason that I know of is that Erlynmeyer flasks are normally pyrex, or made from a substance which means you can heat it on the stove so you can reduce the risk of getting unsanitary conditions in your starter.