The US
DTV standard was chosen assuming reception would be with fixed outdoor antennas.
If you have missing stations and/or stations that won't maintain a lock and suffer from freezing or dropouts at certain times of the day, an outdoor antenna will nearly always help.
If you get all your stations now and don't have freezing or dropout problems, an outdoor antenna will not improve the picture quality. You either get
DTV perfectly, intermittently with dropouts or not at all.