what 's the reason of "nest.cat("all").sadd(String.valueOf(JOhmUtils.getId(model)));" ?

[lvye351]

redis\clients\johm\JOhm.java

// always add to the all set, to support getAll
nest.cat("all").sadd(String.valueOf(JOhmUtils.getId(model)));

it cause :
"SADD" "User:all" "2"
"SADD" "User:all" "2"
"SADD" "User:all" "2"
"SADD" "User:all" "2"
...
in which situation,it is used,who can show one ?
i can not understand the comments "always add to the all set, to support getAll"