Plastic is widely used, but it doesn't have to be as widely used. Certainly, there are many applications where it's essential (medical, food safety), but a lot of applications where it just happens to be the cheapest and/or most convenient option. By raising the cost we are leveling the playing field for other options.

The most pressing example is fast fashion. The clothes are too cheap, incentivising use-and-bin behavior, and they also don't last because they're cheap synthetics. Cotton is relatively more expensive, but at the very least lasts longer. The race to the bottom must be halted.