The main tasks of NPCs called Guides are to show newcomers where they can find the bank, depo, or any other specific locations (shops, adventurer's guild, offline trainers) on the map. Guides have a lot of to say about their cities - for example, something about hunting grounds, quests. They may escort you straight to the depo when you're below level 10.
For higher levels guides are useful to mark a map in newly implemented cities (like recently Issavi) but what is most important for higher levels are WORLD CHANGES.
When you ask the guide:
Player: keyword or change
Guide Kunibert: Valid keywords are: Horestis, Mage Tower, Master's Voice, Swamp Fever, Thornfire, Twisted Waters, Awash, Steamship, Horses, Overhunting, Demon War, Deepling or Hive.
You can choose world change and know the current status.
To know how Tibia NPC Guides work I suggest you read the whole transcription, for example, Rathleton's guide - Kunibert.
https://tibia.fandom.com/wiki/Guide_Kunibert