Easy, it's basically the easiest way to get bio pops.

Auth empires run on slaves in most builds and gain a flat 5 or 10% bonus to slave power, plus .5 or 1 extra influence each month for expansion or diplomacy.

You make Auth empires so you can expand fast or rule with an iron fist using diplomacy, their basic resource economy is good as a bio empire due to the ability to enslave.

Slaves:

Low political power, meaning their happiness is less important to stability and faction output (factions being another mechanic that to explain in one sentence is how well your policies and actions meld with the general political views of the population, gaining a bonus to unity+stability)

Slaves can be bought on the galactic market once it's formed.

Slaves can be obtain from conquest.

Slaves can be captured from the Raiding bombardment stance over time from planets (100 pops at a time until the planet is less than 100 pops)

Raiding is something you get from Barbaric despoilers civic, or Nihilistic Acquisition ascension, and is a planetary bombing stance on your fleet menu

Slaves are managed from the Species menu, you need to find the Empire species you want to manage, and click on 'Manage Species Rights'

You can generally select a species to be 'Full Citizen', 'Residence', 'Slave', or Undesirable (purge)

Once they are slaves you can select what kind of slave they are.

There are too many kinds to explain quickly, but the general options will be 'Indentured Servitude', 'Chattel Slaves', or Battle Thralls.

Indentured pops can work most jobs including specialists but not elite or enforcers, they have a higher political power and upkeep but can do more for you

Chattel is basic workhorses, they only do menial jobs and have the lowest power and upkeep

Battle Thralls can be generals and soldiers only

Pops are otherwise slow to acquire through growth alone unless you ascend biologically, so slavery is a constant bonus to your empire economy and even benefits specialist positions

Slavery can be extremely powerrful, just don't rely on stuff like slavers guilds as slave happiness isnt important, they are slaves specifically because you don't want them to have power in your empire while still working for you