Just like with technology, we need to determine how common magic and the ability to control it is. Rarity makes it more special, valuable, and feared, while it being common tends to get it taken for granted. We don’t need to explain the prevalence unless it has changed in the setting, such as hardly anyone having the ability and suddenly almost everyone does. We can just state it’s one way or another, but how do we decide what we want? By understanding the impact on setting and stories.
The more common magic is, the more it affects life (and stories), just like Earth technologies today. Do we want it everywhere? Magic will dominate. If we skip thinking this through, we risk a mistake: magic can resolve plot problems when we didn’t want it to. Let’s say we plot a story where the characters need to get somewhere in two days by horse, and along the way, someone joins their group, and they acquire both important information and an item. Then we decide magic is pervasive and there are magic gates they can use to traverse this distance in ten seconds. Now it doesn’t make sense that they’ll go by horse, which they don’t learn the information and don’t have the item or companion. Now we must fix our plot. Avoid this by working out the magic prevalence first.
If magic is rare, we get the usual fantasy setting of a few people being able to do it. Most life happens without magic, which is used by individuals mostly for their own benefit. If magic is common, it’s likely been commercialized so that society benefits from it. Just as we have engineers who work for a settlement/government, we might have wizards whose jobs involve using magic for the benefit of all. We must decide the degree of this. An easy way is to think about modern technology and its advantages, and then determine a magical equivalent. It helps to divide life into categories, such as farming, manufacturing, government, communications, and daily life. What can they do with it?
Magic items are a primary result of this. Think of appliances in your life and whether something comparable exists, but don’t do a one-for-one switch where the only difference is whether there’s a plug for electricity or not. A magic toaster likely works and looks differently than an electric one. We’re not just replacing the energy source, but reimagining how the end result can be achieved via an item with the same purpose. Combining purposes is one way to achieve this. We may have a toaster, microwave, stove top, bread maker, panini press, and oven in our house, plus a grill out back. Do they just have one thing and the spells are the difference?
Instead of items, we can have wizards who use their skills “in real time.” An example would be that there are no magic gateways to travel between locations, but there are wizards who excel at transporting others and they work for the government doing this, stationed at specific locations, just like a train depot, for example. They’d likely have insurance via their employer for accidents. Do they have an 8-hour workday? Imagine the fatigue. They may work in teams for that reason. We could have different spell types for this, each named, with some being preferred and more expensive. We can do this exercise with any aspect of modern Earth life.