Why Was Tanistry Important?

Tanistry, a custom among various Celtic tribes

—notably in Scotland and Ireland—by which the king or chief of the clan was elected by family heads in full assembly. ... This system of succession left the headship open to the ambitious and was a frequent source of strife both within families and between clans.

What was the role of the tanist?

In this system the Tanist (Irish: Tánaiste; Scottish Gaelic: Tànaiste; Manx: Tanishtey) is the office of heir-apparent, or second-in-command, among the (royal) Gaelic patrilineal dynasties of Ireland, Scotland and Mann, to succeed to the chieftainship or to the kingship.

Is Tanistry elective good?

For most cases, Tanistry is the superior version of elective (risk of your dynasty losing its authority is the risk that ain't as present here). Also, it would be a waste to not use the law when only the Celts cane have it. You won't be able to play with it in non-Celtic playthroughs so use the opportunity.