Mules are what you reach for when you need to look like you made an effort without actually tying anything to your feet. Slipping them on at the door on the way out, no bending down, no fiddling with a buckle, just step in and go.
They have real limits, though, and it is worth being honest about them before you fall for a pair on a screen. Anything backless needs a foot that stays still, and mules are hopeless for a long walk or a day where you are up and down a lot, because the heel has nothing to hold onto and your foot slides forward with every step, especially if the sole has any shine to it. Stairs are where they catch people out, going down rather than up, because there is a split second where your foot wants to come out of the shoe and your toes grip to stop it happening. Wet pavements are a similar story. If your day involves a train platform, a long corridor, or anything where you might need to move quickly, they are the wrong choice and no amount of liking the shape will change that.
In the shop, check the depth of the vamp, that front part that sits over your toes, because too shallow and the shoe will not hold your foot in place at all, too high and it can dig into the top of your foot when you flex. Walk a proper distance in them on the shop floor, not just a few steps, and pay attention to whether your heel is lifting with every stride, because a little lift is normal and a lot is not. Check the insole for grip too, some have a slight texture or a strap-free footbed lining that stops your foot sliding forward, others are smooth as glass and you will feel it within ten minutes. They suit the warmer months best, from late spring through to early autumn, worn with bare ankles, and they are at their best for things like lunches, work when you are mostly seated, or an evening where you know you will be sitting down more than standing. They are not for anything where you are on your feet for hours, and they do not want to be paired with thick socks, tights, or anything bulky at the ankle, which crowds the shoe and fights the whole point of it looking neat and bare.
A good pair of mules makes an outfit look finished in about four seconds flat, and that is exactly why I keep buying them despite knowing full well I will not wear them anywhere that involves real walking.