Steeper ST angle allows for a shorter chainstay, and therefore a shorter wheelbase - or alternatively a larger tire size, if the builder so chooses.

A ST angle that is too far off from the norm could screw up the relationship between the chainrings and the front derailleur.

A steeper ST angle also "pushes" more of the TT out in front of the saddle, so if you're comparing measured TTs among your bikes, other things being equal, your bikes with steeper ST angles should have shorter measured TTs, or you'll need a shorter stem. There's a rule of thumb for this, but I don't have it here on the tip of my fingers.