starching adds body and stiffness to your fabric making it easier to get accurate cuts, blocks go to gether (crisply)
some fabrics that are thin, haven't much body, or if accuracy is really important starching really helps. as far as
all the time? no...only when necessary.
i keep a spray bottle of water on my ironing board, when pressing i spritz with water, if it is not enough then i grab the starch, but one can of starch has lasted me over 3 years.