When I do my weekend cooking or baking, I plan how things are going to be cooking in my oven. When your oven is on, you are using electricity and heating up your kitchen (bad in summer, not as bad in winter). So, if you can put several things in the oven at once, you will be saving not only money, but time as well.
In the photo below (excuse the mess my poor oven is in) I've got a lasagna baking as well as a loaf of bread, a head of garlic roasting and a large sweet potato we will eat later this week. I'm sure I could have stuck something else in there like a pan of brownies or something. You get the idea though. Sometimes it takes adjusting (not everything cooks at 350 degrees), but you can usually get it to work pretty well for you.