When it comes to plastic-bag storage, there are a lot of good reasons to remove as much air as possible. Marinating in an air-free plastic bag helps better distribute marinades around food. Excess air causes oxidation that can develop into off flavors or promote spoilage. Air pockets can exacerbate freezer burn in the freezer and slow down sous vide cooking. Removing that air is simple to do with a vacuum sealer, but what if you don't own one or don't want to use the expensive bags for a relatively simple storage or cooking task?
Here's a quick, easy, inexpensive option called the water displacement method. All you need is a zipper-lock bag and a tub or pot of water.
I first learned about this technique when Dave Arnold demonstrated it to me as an alternative to vacuum sealers for sous vide cooking, but it has far wider applications.
To do it, you start by placing your food inside a zipper-lock bag, then seal the bag, leaving just the last inch or so of the seal open. Next, you lower the bag into a pot or a tub of water. As the bag gets lowered, water pressure will push air out of the bag through the small opening you left. Just before the bag gets completely submerged, seal off that opening and pull the whole bag out of the tub.
Ta-da! Food that's sealed in a nearly air-free environment, no special tools required.