Q:

Why such long list of users and groups in my system?

On my laptop $ cat /etc/passwd | wc -l results 42!
And cat /etc/group | wc -l results 70!!  
The only user of my laptop is myself and there is only an extra user named "guest". But why such large number of users and groups are available?

A:

In short: privileges separation. A lot of different services and subsystems have their own user and group. It helps to fine tune access rights. Such system also servers to enhance security - if an intruder, or app/script that has malicious code in it, would make it to be recognised as that user, not all of the system would be compromised.

