If as a society you defeat a bunch of people in combat, you will very probably have lots of prisoners.
If you don't have a huge food surplus, you sort of have three choices. Either you kill them, you let them go, or you enslave them.
Letting them go, for a society that is fighting for something other than lines on map, and in an environment where they can go back to being combat effective in a short space of time (say, spears and leather vests) is not always a good option.
Killing people in cold blood is apparently quite hard for lots of people.
Slavery might be seen as the least bad option.
Somewhat from memory, the term "Slave" came from the term "Slav", the people living in bits of central Europe. Norse "Vikings" would do a grand tour involving visiting Russia, sailing down the Dnieper to the Black Sea and then Constantinople, sell all the stuff (furs slaves, etc) for gold, silks and spices, buy a new boat, and sail back to Norway/Sweden via the North Sea. Originally, they might have captured some of these people in battle, but after a while, intentionally taking slaves to sell for gold was a popular way for a young man of good family to get his start in life.
I did a fair bit of reading of the sagas and as much history as I could find about Vikings (about 15 years ago now) and I'm aware that some of the currently accepted stuff has changed, but I'm not entirely sure in which areas.
My memory is that when the Vikings took slaves home (though this may have been specifically Iceland), there were accepted rules of treatment. The owner had to set a slave price (being the price paid, or the price the owner could have got) when they took the slave, and they had to give the slave enough land to run their own garden plot. After the slave had done their usual hours for the day they had to be given time to work on their own plot. The produce from that plot could be sold and the slave could keep the money (perhaps as credit) towards buying themselves out of slavery.
It was said that a very dedicated and competent person could buy themselves free in a year, an average person in two, and someone with any capacity to work at all could buy themselves free in three.
After they were freed, the previous owner owed them some duties. Since most of the laws depended more on patronage and friends than legal institutions, to get any sort of justice, you needed someone to support your cases in the law courts. Since someone brought to the area as a slave didn't have those networks, the person that brought them there was required to do it for them.
This is a very different picture of slavery to the standard Africans exported to the US model, and I sometimes think of Amarrian slavery that way when I think of Holders who aren't being bastards (except for the whole buying themselves free thing).
[Note that I couldn't give you references, and my memory is going... Oh, is that the time? Lunch!]