If you cook sous-vide for immediate serving (cook-serve), a vacuum sealer is not needed. Using good quality re-sealable freezer bags with the water displacement method works well for sous-vide cooking temperatures.
A vacuum sealer is recommended if you tend to store food cooked sous-vide for consumption at a later date (cook-chill), require compression of certain food or use other techniques such as flash-pickling. We recommend exploring cook-chill at a later stage once you become more familiar with the cook-serve method.
The main purpose of the vacuum (or just an airtight bag) is to ensure an efficient heat distribution from the water bath to food for even cooking, because air is a poor conductor of heat.