Here are some thoughts from my experience working on The Sims 1:

Even if you don't sit in a chair or less functional piece of furniture like a fish tank, it also serves as an obstacle that affects everything you do in that room, by participating in the A* "maze game" of getting the characters from one place to another and possibly making parts of the room inaccessible.

In more typical shoot-em-up games, furniture also provide cover for ducking behind when players are shooting at each other. Fallout 4 is a lot like The Sims with weapons and radiation, in that respect.

By adding a few strategically placed pieces of furniture, you can increase the time it takes for a character to take a shit and shower in the morning, so they miss their carpool, lose their job, and then finally have to stay home all day and actually have time to sit in the chair.

One of the early working names for The Sims was "TDS: The Tactical Domestic Simulator". As the design evolved, it became clear that building the house and arranging the furniture was an important part of the game, but it was difficult to come up with a way to objectively evaluate the "feng shui" of the architecture.

Will Wright realized that the simulated people could actually play that role in an indirect way by how the physical geometry of the architecture and furniture placement affected their use of time, space, and resulting happiness, etc.

If you can't make it to the bathroom in time because somebody's temporarily blocking your way, you crap your pants and dump a blue puddle on the floor, your personal hygiene score goes down, the room score goes down, and somebody has to spend their time cleaning it up.

That made it less of a materialistic game of simply buying as much stuff as you can to make your characters happy, and more of exercising restraint and leaving enough negative space and open floor and adjoining doors, that the characters can get on with their days, get from place to place without bottlenecks, invite more friends over, mingle around, dance and socialize with each other.

In order to use the toilet, the character has to be in the room alone, so it doesn't make sense to put a toilet in the living room in front of the TV for efficiency's sake, unless you live alone. If other people are in the room when they want to use the toilet, they will attempt to "shoo" them out of the room for privacy.

Here is one of my favorite bugs from unintentional emergent behavior, involving the toilet and the maid:

A dude walks into the bathroom, pixelates his crotch, pulls down his pants, sits on the toilet, and proceeds to take a nice leisurely dump.

But as soon as he pinches a deuce into the bowl, the toilet immediately clogs up and starts desperately advertising that it's dirty and needs to be plunged.

Of course that attracts the attention of the maid, who at the time was blissfully uninhibited, and cheerfully wanders into the bathroom, then reaches around behind her to whip out a plunger from "hammerspace".

Then she helpfully thrusts the plunger into the toilet bowl through the still-squatting dude's pixelated crotch, with the long hard plunger handle sticking up at full attention, and starts vigorously jerking the handle up and down to unclog the toilet while the lucky dude was still sitting on the toilet!

I'll leave it to your imagination what it looked like was really going on...