It fulfills the needs of capitalism by ensuring space is used to its maximum utility. It improves the health of cities by increasing its tax base and proving new safe housing for families with well paying jobs. It's good for residents who see improved quality of life and better jobs.
Those who are "harmed" by gentrification are really no worse off than before. They just have to move. And because the neighborhood has improved, by definition they were the problem. If you aren't actively engaged in making your neighborhood better, you are making it worse.
Governments are inherently wasteful and churches have other agendas. Charities are either money laundering fronts or inept or too small. The best, and really only way, to fix poverty in inner cities is to gentrify.