Oven Kashering

Question

Tell me why I should kasher my oven (other than the reason that treif things may have been cooked in it prior to my ownership)? I mean, once it's kashered, can I ONLY cook meat OR dairy dishes in it? I.e., I have to choose to cook only one type of food in it from now on? It doesn't make sense to me. I use the same refrigerator for my meat and dairy. However, I'm supposed to have two sets of dishes? If I wash them in hot water, what makes them less accommodating than the one stove or refrigerator? BTW, this is an AWESOME forum. Thank you SO MUCH for its being!