What's the meaning of "resources external to the SQL Server computer are needed" in Microsoft's SQL documentation?
This documentation page gives an explanation for those curious enough to scroll down:

A Managed Service Account (MSA) is a type of domain account created and managed by the domain controller. It is assigned to a single member computer for use running a service.


Virtual accounts [...] are managed local accounts. [...] Virtual accounts cannot be used for SQL Server Failover Cluster Instance, because the virtual account would not have the same SID on each node of the cluster.

Emphasis above is mine.
In other words, virtual accounts are local to one server and cannot control resources on other servers, while managed service account are domain-scoped and can, therefore, be given access to resources on multiple servers.
For a standalone SQL Server instance there is no difference in functionality. In a cluster, however, the account should have authority to access and/or modify various resources on more than one server, so a domain account would be appropriate.