A pool is a way of organising your backups together. For example you may keep your incremental backups in one pool and full backups in another pool which may be useful if you have a one month expiry on your incrementals and a one year expiry on your fulls - otherwise sending them to the same pool would see the incrementals waiting around for a year for the full backups on the same tape to expire. Or you may keep backups from one site on a different pool to another site which may make them easier to find or perhaps reduce the number of tapes you need to find for a restore. Or you may apply encryption to one pool of tapes. Or you may make an extra clone of one pool of tapes.
Its simply a way of keeping similar data together.
Important point - once labelled in a pool a tape cannot be moved to another pool without relabelling (which will remove all data from that tape).