I found this on wiki..

"While traditional anticonvulsant drugs are predominantly antimanics, lamotrigine is most effective for preventing the recurrent depressive episodes of bipolar disorder. The drug seems ineffective in the maintenance of current rapid-cycling, mania, or depression bipolar disorder; however, it is effective at prophylaxis or delaying the mania, depressive, or rapid cycle".

I guess it's just something that is like "I'm gonna put you on this in case it works and gives me time to treat the manic depression".

But don't take my word. Personally, I'd rather be on the lithium alone if the lamicital causes such sedation. I like my brain zaps..