Source : Webster's Revised Unabridged Dictionary (1913)

Subserve \Sub*serve"\, v. t. [imp. & p. p. {Subserved}; p. pr. &
   vb. n. {Subserving}.] [L. subservire; sub under + servire to
   serve. See {Serve}.]
   To serve in subordination or instrumentally; to be
   subservient to; to help forward; to promote.

         It is a great credit to know the ways of captivating
         Nature, and making her subserve our purposes, than to
         have learned all the intrigues of policy. --Glanvill.

Subserve \Sub*serve"\, v. i.
   To be subservient or subordinate; to serve in an inferior
   capacity.

         Not made to rule, But to subserve where wisdom bears
         command.                                 --Milton.

Source : WordNet®

subserve
     v : be helpful or useful