It's a pretty popular search command and it is used in all sorts of situations. Below are some really cool searches that use fillnull along with other search commands.

User Search Restrictions and Last Logins

| rest /services/authentication/users splunk_server=local
| table title realname email roles 
| mvexpand roles 
| join roles type=outer 
    [| rest /services/authorization/roles splunk_server=local
    | fields imported* title srch* 
    | fields - *Quota *TimeWin *capabilities 
    | rex mode=sed field=srchFilter "s/^\*$/true/" 
    | rex mode=sed field=imported_srchFilter "s/^\*$/true/" 
    | eval search_restrictions=if(imported_srchFilter="","( ".srchFilter." )",if(srchFilter="","( ".imported_srchFilter." )","( ".srchFilter." ) OR ( ".imported_srchFilter." ) ")) 
    | fields - srchIndexesDefault 
    | eval srchIndexesAllowed_new=mvjoin(srchIndexesAllowed," OR index=") 
    | eval index_restrictions="( index=".srchIndexesAllowed_new." )" 
    | table title search_restrictions index_restrictions 
    | rename title AS roles ] 
| stats values(roles) AS roles values(search_restrictions) AS search_restrictions values(index_restrictions) AS index_restrictions by title,realname,email 
| rex mode=sed field=search_restrictions "s/\( \)//g" 
| eval search_restrictions=mvjoin(search_restrictions," OR ") 
| eval index_restrictions=mvjoin(index_restrictions," OR ") 
| eval realname=upper(realname)
| rename title AS username
| eval restrictions=if(search_restrictions="( )",index_restrictions,search_restrictions." ".index_restrictions)
| table username email username roles restrictions
| search username=* 
| rex mode=sed field=restrictions "s/^ OR //g" 
| join type=outer username 
    [ search (index=_audit "login attempt info=succeeded) OR (index=_internal "GET /splunk/en-US/account/login") 
    | stats max(_time) AS "last_login" by user 
    | rename user AS username ] 
| eval days_ago=floor((now()-last_login)/86400) 
| eval last_login=strftime(last_login,"%Y-%m-%d %H:%M:%S") 
| fillnull last_login value="Never"




Shows a table showing the inherited search and index restrictions for each user, represented as an SPL string. In certain cases it may not be 100% accurate, but it covers most instances that I have run into. NOTE: does not account for `searchFilterSelecting = false` defined in authorize.conf