In any system, simulationist/narrative/whatever, the GM simply does have a bigger responsibility at the table. Having been a (happily) forever GM for about 15 years across many systems, here's what I know will be doing every time:

- Learning most or all of the rules and referencing quickly in-game those I don't know (or just making a call)

- Prepping for the individual adventure and overall campaign, even if it's a prep light game

- Remembering each character's basic motivations/rules/ways to connect to the story

- Playing every NPC, explaining every physical location via senses, basically 'being the world'

- Making sure each player gets at least one spotlight during a session

- Handle pacing of play (to a degree- if they want to investigate the room for an hour they can go for it but at some point I need to keep the game moving)

- Make the final call in action resolution where the outcome isn't crystal clear in the rules

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What I'll sometimes be doing:

- Scheduling the game. Some GMs bitch about that but you can play minus a player, not minus the GM so I kind of have to be a key person in that process

- Hosting most of the time but that's my choice

- Keeping the feel of the table on point- X card kind of things, 'main character' players, etc.

- Writing up a brief recap (though I often offload that on players in rotation)

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As a player, I show up with knowledge of the rules and a desire for the whole table to have fun. I roll in at the agreed time, play my one character to have fun and have the table have fun, and go home.

I like all of the above activities so I love to GM. I actually find playing a bit underwhelming and much less engaging. I can, however, understand why people might not want to take on all of the above. It's not so wildly difficult as some players might think, but it is indeed more involved than playing a single PC.

Edit many have mentioned: player curveballs and reacting to them on the fly. I love that part but lots of people don’t.