Hi Hannahc,
Going lighter is something hairstylists do often recommend to our clients who are going grey and are naturally dark.
Blonde hair tends to look mousey and goes darker before turning to grey.
The reason is this,... as you age your hair looses pigment before it turns grey,thus looking washed out, dry and lackluster . Adding highlights does look softer against the skin because more often than not dark coloring products tend to look...well ...artifical and TOO dark, especially on someone in their 40's thru 70's, because hair color(natural) has so many colors going on.Many shades of reds, browns etc. to create YOUR natural color.
While I think you are still young enough to wear dark hair(and the 20 something crowd can go very dark), adding some highlights breaks up that solid heavy look and adds dimension ,and some softness because of the changing skin color we go through also as we age.Very dark hair does tend to throw shadows on our face.
I tell my client's to think of hair as an ACCESSORY, it is part of a TOTAL look.
Marilyn Munroe would not have been the blonde bombshell if she did not have the TOTAL package. That means hair, clothes body,dark eye liner, red lips.etc. Otherwise she would have just been another bleached blonde :)
Your hair is a huge part of your total look, so if you enjoy it dark for now, that is fine, re-evaluate in a couple years about the highlights:)
Hope that answeres your question:)