there are many reasons - biblical, social, familial, etc - but the one i find most compelling is that church is a place where we surrender control.
at church, as part of a community of believers, we trust that the holy spirit speaks to us through:
music we might not like
a sermon we did not choose
people we might not know
a time and place that is beyond our control
an experience we would never have designed
strangers we would never have met and would never have been forced to confront
christian men and women who can be prompted by the spirit to speak things into our life that
we might never choose to hear
scriptures and songs themed towards issues we may never have otherwise considered
the needs of others about which we would have remained ignorant
in short - church is a place where a million things happen...sometimes randomly, sometimes chaordically, sometimes intentionally...and the spirit works through all of those to penetrate our lethargy, our apathy, and our indifference.
when we come to church with an open and receptive heart, we are changed every time.
that's why we should go to church.