In my opinion you almost never need the latter and people use them when they really just want global access, and using a “Design pattern” to do it feels like good design while “global bad”.

There are very few cases where limiting to a single instance is necessary. If you only want one instance then create only one instance, but there’s rarely a good reason to make it only possible to have one instance and it’s not uncommon to later need more than one instance. For example, loggers are often given as an example of a singleton, because you only need one, right? Until you want to have different loggers for different purposes or destinations.

So if you don’t need to limit it to only one instance and are only using the pattern for global access, well then there are other options that look very similar but without the single instance rule. The ImGui/OpenGL style is like a singleton where you can only have one instance active at a time, but you can create multiple contexts and switch between the active one if you need (it may not be the nicest API if you need to do that, but it at last makes it possible). But simply making a global instance of a class (eg as is often done with loggers) is in my opinion perfectly ok too.

Making a singleton still makes a global and so is no better design than global are: the same caveats apply to singletons as normal globals (ie you should avoid them), so just make a global and don’t pretend your design is somehow cleaner.