I've been getting B12 injections for a couple of years now and the main reason I keep coming back is for the boost of energy. I sleep better and my mood is staaaaaable.
The minute I get a B12 injection I get crazy increased energy levels. Other benefits include (but are in no way limited too)...
+suppresses appetite (I literally have to remind myself to eat)
+helps control anxiety and depression
+helps with those ADD moments and gives you more clarity
B12 deficiencies can include depression, anemia, memory loss/failure, low blood pressure, mood disorders.
B12 has been prescribed for energy and depression or to help with cognitive functions.
I'm not stopping anytime soon. I get these injections almost every two weeks (or more) ... so much so that my holistic doctor gives them to me to safely inject myself.
Obviously talk with your doctor before starting anything new, but if you're experiencing any of the above symptoms, you might be B12 deficient.