Short answer: Sessions are unpredictable.

Long answer: As a game supervisor I can anticipate a lot of what the players migth do, but I can't prepare for every eventuality. The best example would be combat, it's a part of every system you play and it can take a while. Stopping a session and leaving a fight unresolved isn't fun for anyone, so I'd rather finish the fight and conclude it, pick up right after the fight on the next session. Another example is when you've finished a quest and just need to collect your reward. Doing that doesn't take up much time. This also contributes to a sense of fulfilment instead of just stopping a session because time ran out.

Does this mean that you'll always get to play longer, no, a session can be concluded in a timely fashion without the need to extend a session.

Before a session:

• I create a piece of website where you can find information about your campaign.
• I create new characters or lookup pre-mades.
• I prepare challenges for the party (puzzles, combat encounters, villains, etc.).
• If it's a follow-up session, I read the summaries that I wrote.

During a session:

• I explain rules when needed.Leg ik regels uit indien nodig.
• Or make a decision to keep the game going, instead of interrupting game-time to look something up.
• I describe any loction you might visit.
• I play any character you want to talk to.
• I make sure each player gets the opportunity to play their character.
• I help new players by giving them advice on what they can do (unless they're playing a Bard, I hate Bards... :D).
• I make sure that players who behave badly, as a player, know their place. A game needs to be fun for everyone.
• I keep an eye on the game-time.

After a session:

• I write and upload a summary of the session to your private website.
• I'm already thinking about new things to throw at you during the next session.

In between sessions:

• I create more pre-made characters.
• I create maps.
• I read up on rulings that I might've forgotten about during a session.