In baking, butter vs oil makes a huge difference in both taste and consistency. Since there is so much to go over for that and I am not super knowledgeable about it, this post will be mainly on other uses. I will also be only talking about olive oil, because that is the only oil I use.
Whether you are frying or heating food up, using some kind of oil or butter is generally recommended. Cooking some foods without some kind of liquid can cause burning and are also not good for the pan. Re-heating leftovers without liquid can make them come out really dry, and stovetop is way better than microwave for almost everything except ease of use and time.
When I cook chicken, cauliflower, onions, eggs, virtually anything stovetop, I do so with a bit of butter. Instead of brushing baked goods with egg wash, I usually use melted butter. It’s easier to apply and mix, and to me it tastes much better.
For the most part, it’s personal preference on which one to choose. I’d use oils for frying, just because it gets nice and crispy. For things like steak and chicken, the butter adds flavor as it cooks and makes it nice and juicy instead of making an outer crust. This is also affected by temperature, so don’t forget to take that into account as well. I think oil tends to splatter more than butter, but that is not a huge deal since both will splatter anyway.
With sides for bread, there are a few options, some of which are butter and oil. Garlic bread continues to be one of my favorite foods, whether the butter is dipped, spread, or cooked in. Oil and vinegar is a great dipping sauce for bread, though I’ll admit, it’s not for everyone. There’s a lot of variation, with mixing ratios, flavors of both oil and vinegar, as well as type of bread for soaking.