Annual surveys (whether you send them once, twice or more times) show a general overview of engagement while pulse surveys allow you to follow specific items in shorter periods of time so you can check them more regularly, follow trends, and react on time.
We will take the coronavirus situation as an example. In Improv3 we created a short questionnaire that contained only multiple-choice and open-ended questions specifically focused on the new situation where feedback from employees was important and necessary for the organization and wellbeing of employees.
But pulse measurements are also used if you want to track changes and trends:
After or during some projects and changes in the company
After a certain time period
After the implementation of action plans
Of a target department
Of 2-3 dimensions that had bad results