They were technically slaves, in a gray area between slave and freedman. Somewhat like an indentured servant, however it’s very unlikely they would be freed if the Persians won.

They were the lowest class of Spartan “society”, basically residents of the areas surrounding Sparta that were under Spartan control, and made up the majority of the population of said Spartan territory, Laconia and Messenia. In “exchange” for being protected from invasion etc, they were made to perform mainly agricultural duties and some were taken on as what we know as squires in battle.

Perhaps a kinship was formed between solider and squire, but most likely it was them preferring to die in battle than be subject to whatever, probably torture, the foreign invaders might have in store for them.