It's all about minimizing the risk of data loss. The risk is always there, but some DB are much safer than other. For example, Server crash is a much rarer iss

It's all about minimizing the risk of data loss. The risk is always there, but some DB are much safer than other. For example, Server crash is a much rarer issue than process restart. In Redis, if the process supervisors (systemd, docker, k8s, etc..) decide to restart your Redis process for whatever reasons, you risk losing ACKed data, because Redis ACKed before the data is written to disk. In a safer database system, the data is not ACKed until it can safely survive a process restart. Furthermore, many database systems can be configured so that the data is safely persisted in multiple servers before ACKing, and therefore survive even a server lost incident.