I have to admit that you are nerdier than I (am)! -- I was joking. Besides, I now believe nerdy is truly beautiful! And I am serious with the latter statement. :-)
我做了一点功课。没有在权威语法书上找到答案，是在网上找到的解释。关于besides的这个用法，大致是这样说的：
Besides is highly used in spoken English. It is the colloquial equivalent of "in addition to", "moreover". （We should try not to use "besides" in formal writing.） It has to always be paired with "also" as shown in the following sentences:
(1) Besides driving us to the Mall, she also bought us ice cream.
(2) Besides being under the weather, she was also frustrated.
(3) Besides being a politeness strategy, hedging can also be a means of joining an ongoing conversation.
You can break the above sentences each into two sentences. But then you can't ommit the word "that" following "besides":
(1) She drove us to the Mall. Besides that, she also bought us ice cream.
(2) She was under the weather. Besides that, she was also frustrated.
(3) Hedging is a politeness strategy. Besides that, it can also be a means of joining an ongoing conversation.
还要继续琢磨，回头要是找到更权威的说法，我再更新与你。
