My newbie tarot friend is in love with the base or "shadow" card. I now call her the Shadow Queen.
When she's doing readings, she'll even look at the base card first before the rest of the reading...LOL. She's a bit much.
I don't do base/shadow cards myself, although reading with this friend has made me look at the bottom card of the deck here and there. I think the idea behind the base card is that it gives you an additional clue, or tells of something that it's in the "shadows"...maybe unknown to you or the situation. Some call it "hidden factor" card.
For example, let's say you do a reading about a significant other. You ask, "What will he do next with regards to me?" For the sake of this example, let's say you get Two of Cups and Ace of Cups. You'll read, "Wow, he'll express his huge feelings for me, how wonderful!" Great. But then you also look at the base card, and the card is, 4 of Pentacles. Now don't anyone attack my views, I'm just giving an example here...
So maybe the base card is saying, though he may express his feelings for you, he actually will turn out to be, or is feeling, quite possessive towards you and that might be what's prompting the big, gushing expressions of feelings. It's that little "unknown" piece of info that you wouldn't perhaps have if you just looked at the cards drawn in your readings alone.
The base card is not essential (in my eyes), but some people swear by it. It all boils down to developing your own reading style and finding if reading the base card does, indeed, help you with the reading, or if it just confuses you more.
Hope that helps!
GA