Trees provide an eco-service:

• They sequester carbon: big and fast-growing trees -- oaks, elms, redwoods, etc. -- will give more bang for the buck than small or slow-growing trees such as crape myrtles or many other ornamental species. ​They take up more carbon as they build more mass.
• Sequestering carbon helps mitigate global warming, but trees also provide shade
  • They help breakup hard landscapes and provide respite - see any schools lately with playgrounds that consists solely of chainlink fences, and hot concrete floors? Help us get some trees to the youth at those schools.
• They reduce storm-water run-off
  
• They  improve air quality and offset air pollution - some studies show that for every 10 trees 1 automobile is offset
  
• The more green a neighborhood is, the more walking and cycling is encouraged - thus reducing CO2 emissions due to the reduction in the number of car journeys .
  
• Trees increase property value and reduce crime
• Trees are essential for human health - science has shown that levels of cortisol (the stress hormone) goes down with more trees  and we know that stress drives bad health.
  • Having more trees provides a sense of calm and improves our mood

More benefits of street trees can be found here - linked to their sources: ​https://caufc.org/why-urban-forests/