In dungeons or in front of a boss's door, you can summon an ally to help you. This feature has been available in souls games for a long time but no one could guess the circumstances that led to the genesis of its creation. During an interview conducted by The New Yorker, a few days after the release of Elden Ring, Hidetaka Miyazaki explained what made him create this multiplayer feature.
A car breakdown story
Miyazaki explained that during a problem with his car on a hill in the middle of winter, while he was looking for help, a group of individuals came to help him. He didn't know them at all but they still helped him push his car to the top of this hill to then disappear after the task was accomplished.
This personal experience gave him this idea that the Souls should have this summoning function when the need for help arise. Much like this group of individuals, players can arrive when the player needs them, and they leave once the fight is over.
On Elden Ring, this functionality could not have been added but the developers thought that it would be beneficial for all players to leave it.
The spirit summoning system was also introduced with Elden Ring, and it still maintains this same philosophy: you summon an ally who will assist you during a difficult part of the game, such as a camp full of enemies or a boss, then disappears once the work is done.