Well, I'm throwing in cargate as a necessity because without cars anything they shoot in a city or suburb would have to be digitally edited to remove the vehicles, or they would have to remove vehicles from the actual areas where they shoot, which still wouldn't explain the curbs, pavement, and white lines marking parking spaces. They could still do indoor shots without cars, and could rely on those for some planets, but it would be hard to present city shots without the street scenes and obvious roads between blocks of buildings. It also frees them to use tons of stock footage of Earth cities from around the world to reflect how the different countries recreated their home environments on other planets.
So my thought is that not only is it cheaper to leave the cars in, they could also support the story in clever ways when needed, and be ignored when not. They'd also provide a useful and obvious benchmark for the relative technology levels and freedoms on different planets. If the cars are all different and modern looking, it's a good bet it's a reasonably advanced world. If they're all small, black, and similar it's probably not. If there are almost no cars and the buildings are run down, it's an obvious clue that not every planet or people really advanced much. And occassionally they hit the planet with flying cars, for contrast.
It's not really a feature, but more of a bug fix that lowers production costs, and frankly, most viewers wouldn't imagine a future without convenient high-speed personal transportation. Of course for many episodes the SGC people might not even make it out of the gate complex, because many societies might be more than paranoid about what might come through the gates, including humans from planets they despise. I'm sure Jack's arrival at the Nation of Islam planet will be interesting.
Cars, especially future ones or special ones, can also make for very interesting action sequences, such as in the Matrix. Stargate mostly relied on small groups of infantry shooting at each other, shooting at bugs, running around, or standing in front of consoles. Hopefully the addition of car chases or crashes, with the twist that some cars are definitely not equal, would expand the program's visuals and change its look while providing something the viewers can relate to. Heck, whole series have been built around advanced cars with talking computers.