Some pension files are rich with genealogical information.
This particular file contains the following gems:
(1) Full name before AND after the Civil War
(2) Reason for the name change
(3) Birth place, date and birth order, mother's name
(4) Mother's slave holder, location of 'plantation'
(5) Father's name
(6) Names of some of those who served with him.
(7) Physical description
(8) name of wife (Picture)

Even if your ancestor did not file for a pension, look for the app of his surviving widow (or a relative).
The pension record of Fellow soldiers could also hold clues to an ancestor's origins, travels, migration pattern - especially if that fellow soldier lived in the same post-civil war town or married into the same family.

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¹ Shaffer, Donald R., and Elizabeth Regosin. "Voices of Emancipation: Union Pension Files Giving Voice to Former Slaves." Prologue Magazine, Winter 2005, Vol. 37, No. 4 . 24 Jan 2006 [www.archives.gov/publications/prologue ...].