The subjuntive mood was more common in "Old English" (Biblical or Shakespearean English) than it is today. Nevertheless, the tense most often conveys a sense of:
Utter respect - Example - in the Lord's prayer, "hallowed be thy name" utilizes the subjunctive in this way. "Santificado sea tu nombre" instead of "Santificado es tu nombre"
Good wishes - Example - "May you have a good day," utilizes the subjunctive in this way. "Que tengas buen día" instead of "Que tienes buen día"
Hypothetical scenarios - Example - A common hypothetical phrase used English is "If I were you . . ." It uses the subjunctive--"Si yo fuera mi hermano, tendría mucho cuidado" instead of "Si yo era mi hermano. . .
Doubtfulness/disbelief - Example - "I can't believe how warm it is this December" would be "No puedo creer que haga tanto calor este diciembre" instead of "No puedo creer que hace tanto calor este diciembre."