1) You can import and export media when other backup or restore operations are taking place (might be library specific) - bigger libraries have interlocks so that other operations cannot take place if manual activity against the storage slots is occurring.
2) You may not have to run an inventory job after changing tapes (again might be situation specific) - if you manually move tapes into or out of slots BE won't be aware of the changes so an Inventory will always be needed.
3) if you eject at end of job, it saves time the next day AND a realtively unskilled staff member can easily be used to change tapes,. If you open up the slots then more time (and trauining level) may be needed.
At a basic level however it is up to you which method you use as the end result will/should be the same.
Note: an eject only applies to a stand alone drive and not a library as the static state for a stand alone drive is tape still in drive, but the static state for a drive in a library is no tapes in drive, all tapes in storage slots