What was the significance of gladiators among slaves?

It is known that in ancient Rome, some of the slaves could become professional warriors who fought in the arena. Such warriors were called gladiators. Therefore, the selection of future gladiators among the slaves was carried out according to the following criteria: physical data and the presence of service in the army (that is, in fact, they were soldiers who were captured after Rome seized their lands). Since gladiator fights were very popular in Rome, some gladiators could achieve great fame. There are sources that say that, in this way, they could get their freedom. True, most of them most often died in the arena.