Mustard will dry out the bread if you aren't eating the sandwich right away. So if I'm packing a sandwich, I'll butter the piece of bread that the mustard goes on to keep it from drying out.
I'll put butter on one piece of bread, mayo or Miracle Whip on the other for a turkey sandwich. Along with some cranberry orange relish. Yum!
If whatever I'm putting on the bread is somewhat spreadable, I don't use any butter - peanut butter, egg salad. But if it is lunchmeat, I'll sometimes butter one piece of bread. If the bread is really fresh, I don't bother.