Since villagers are quite rare, being non-spawnable by the /spawnentity command, and sometimes not being found in untouched villages, they added it so you can get villagers. For mending and stuff. Just right click it to spawn a villager.
People usually buy them for 0.1-0.3 cubits like said above. But cubits are like 2b (2,000.000.000) each so, they're like 200m-600m each. But no one has that crazy of an amount of money, well, not many new players. So they usually sell for cubits.
