I'm wondering how celebrities planting trees in pots along city streets, or anyone else for that matter who plant trees for the environment, are going to help anything. Weren't the trees already growing? Aren't they just transplanting trees? I don't see anyone putting tree seeds in the ground. The mere act of planting a tree is just that...an act.
Yeah, yeah, yeah...we have trees where I live too. But I still fail to see how moving a tree from one place to another has a positive impact on global warming. Talk all you want about photosynthesis and removing carbon dioxide from the air, I understand that process. But if you have Tree X in location 1, and you move Tree X to location 5, you haven't made any progress in saving the world. In fact, the energy consumed to relocate that tree probably leaves you with a net negative environmental advantage, at least for the next several years until the tree matures.

itz becuz plating those trees make space for new seeds to grow at the nursery.

Trees make an urban environment better. They clean the air, provide shade (which, when they're tall enough, keeps homes in their shelter a little cooler in summer, and more sheltered from wind in winter). They beautify an urban landscape that can get very gray. Do they matter to people? You bet they do. When I lived in New York, I was a volunteer licensed to prune and care for street trees (it took a 6-week course to get certified, and if I could find my course book, I could probably come up with a few more reasons why trees are worthwhile in cities, because that was covered on the exam). I used to joke that you could mug someone's granny on the street and people would walk on by, but if you were sawing on a tree to repair a broken branch, you'd get all sorts of people asking you just what you thought you were doing.

And yes, moving trees elsewhere creates demand, so nurseries keep planting trees. There's not a finite number of trees in the world.