The roles of a Webmaster may differ from company to company. But the 3 basic roles are:
1- Build new functionality - ensuring that new features are built based on business needs that delight a user and that will make them want to come back.
2- Security - ensuring that a user's Personal Identifiable Information (PII) such as name, credit card info, addresses, social security numbers etc are protected against malicious cyber attacks.
3- Availability - ensuring that the website or service is 99.9999% available. If a website goes down for whatever reason, it could cost the company millions in lost revenue. This is usually handled by a team called DevOps.
Think of these roles as 3 parts of a circle - all 3 roles are equally important for a website or service to be successful.
Based on this you can answer the second part - which role interests you the most.