Yeast need plenty of oxygen in order to do their jobs. The first thing yeast do when pitched into your wort is take up enough oxygen needed to replicate and energize enough yeast cells to do the job.
You can shake your fermenter for 20 minutes and not get the same amount of oxygen you will get from a 60 second shot of pure oxygen through a diffusion stone.
This is absolutely the best way to aerate/oxygenate your wort. You can buy small canisters of oxygen at any home or hardware store.