If plants benefit from carbon dioxide, why is car traffic something bad?
There are at least two separate answers to your question.  First, with respect to plants needing CO2, they have evolved to deal with the limited amounts of CO2 normally in the atmosphere.  That's really all they need, or "want": adding more doesn't really benefit them.  Think of it this way: you need water to live, right?  And drink a certain amount of it every day.  But drinking large amounts of water can kill you https://en.wikipedia.org/wiki/Water_intoxication and holding you under water for a few minutes certainly will.  (As in the comment above, and even better example would be oxygen: we need it to live, but too much can cause all sorts of problems: https://en.wikipedia.org/wiki/Oxygen_toxicity )   Likewise, plants need nitrates, but applying too much can kill them.  So extra CO2 in the air is of little or no benefit to plants.

The second answer has nothing to do with plants.  It's that atmospheric CO2 traps solar heat, causing the Earth to be warmer than it otherwise would.  Without the extra CO2 from fossil fuels, Earth maintained a comfortable temperature, one that we'd all (and I mean not just humans, but all life) had gotten used to.  Add more CO2, and more solar heat is trapped, making the Earth uncomfortably warm, to the point that many plants &amp; animals can't live where they are, and generally can't migrate.  So you wind up with a lot of dead plants and animals.

Of course there's more going on than this, things that are feedbacks from and side effects of the increased CO2, such as ocean acidification.  If you really want to know more, here's a good starting place: https://history.aip.org/climate/index.htm