Why would you need a tablespace?
initdb initializes the whole server data directory and every file will be in the same disk. With tablespaces you can distribute objects onto different disks when needed. The reasons usually are performance related. For example, you may want to have some rarely used tables on slower disks and high throughput tables on super fast disks.

Another benefit is to store indexes on separate disks so they have separate I/O from the data.

Naturally also if the volume on which the cluster is on runs out of space, tablespaces can be used to allow the databases to grow to other disks. This requires the tables to be moves to the new tablespace, it will not happen automatically.