Automations can update and create things based on a set of rules.
For example; let's say you want all companies that belong to a certain industry and has a certain number of employees to be placed in the same sales campaign. Simply tell the automation to look whenever a company is updated, and check if the industry- and number of employees fields has the right value. If so, add them to the sales campaign.
Instead of relying on a user to do this, the automation automatically takes care of it.