Why would I need more than one API credential?

If you want to separate different tasks that you accomplish through API you can use multiple API credentials. 

For example: If different parts of your org use api credentials you can have one set for each team to use without interfering with each other.

https://docs.streamsets.com/portal/platform-controlhub/controlhub/UserGuide/OrganizationSecurity/APICredentials_title.html#concept_vpm_p32_qqb