More people using public transport means:

1. A wider range of people are able to travel independently. Young people and some disabled people can not drive in most countries.
2. Less raw materials and energy are used to build public transport infrastructure (metals, plastics, coal, gas, lithium ect).
3. More free space in towns and cities for parks, houses and businesses ect that would otherwise be taken up by cars. This also means there will be less distance between parks, houses, businesses ect.
4. Much less death and injuries that cost the taxpayer. Road traffic injuries are now the leading killer of people aged 5-29 years.(According to WHO). Especially with many countries having aging populations, killing the young is not exactly a recipe for prosperity.
5. Less green house gass emissions and environmental damage (for both manufacturing and maintaining public transport).
6. Less environmental damage Using less resources and producing less emissions protects the planet and environment, this allows us to continue to produce things like food and clean water.