This will set the priority for the stack election. If you want a particular switch to be the master switch in the stack you would set the priority higher than the default (which is 1).
Also, you could set a sequence in case of a failure as well. For example, if you had a stack of 6 switches, you could designate one as master (priority 15), a backup (priority 14) and even a tertiary backup (priority 13); this way if there was a failure the election process won't have to determine the master switch.
