--snip--
>My husband told me that
> I have to disconnect 1 battery terminal in order to charge the battery,
> but
> I don't understand exactly why.
--snip--

I have always done this, I suppose because I was taught to do so- however
IIRC it had something to do with potentially getting the wrong cable from
the charger on the wrong terminal of the battery and possibly frying
components upstream; sort of a stop-gap between what might happen to the
charger and the battery and the rest of the vehicle's electronics.
I was told much later that in cars with computers that there was a potential
to harm the computer from charging with the battery cables still connected.
Frankly I have no idea if that is true or not, perhaps somebody else will
speak to it.

Aside from that I've always disconnected the negative battery cable before
doing any kind of electrical work on a vehicle, just as SOP.

Cya,
Robert