Why is a turnbuckle used with pipe supports?

A turnbuckle provides two functions in a piping system. The first function would be to provide a minimum amount of adjustment for any field inconsistencies. The second function would be to provide the minimum amount of resistance when transferring load to the remainder of the support components.

PT&P Catalog – Turnbuckle

Turnbuckle Fabricated And Forged
Forged TurnbuckleTurnbuckle