What is depreciation?? What is cost segregation?

[Aaryan Patel]

Can you guys tell me what depreciation does for real estate investors?  And, what is cost segregation?  As I understand it, you are supposed to divide the building value by 27.5 years, and 39 for commercial real estate.  I'm not sure why we do this, or how this helps real estate investors.  I think it is taxes that are taken away when you sell the property, just for holding it??