Scalability is the benefit that is most relevant in this scenario. Scalability allows a cloud service to handle growing amounts of work by enabling the addition of resources to accommodate increasing load. The cloud's ability to automatically adjust and allocate resources based on demand ensures that the web application maintains performance during the global online event, regardless of the number of participants. High availability, while also a crucial aspect of cloud services, primarily ensures that the application remains accessible with minimal downtime, but it does not directly address the changing load requirements that scalability caters to.