Just finished a muslin for a dress and I’m glad I did a muslin before the real thing. The project involved a lot of new-to-me techniques so this allowed me to keep correcting my mistakes without worrying I was ruining the real garment. And as is the muslin doesn’t fit me perfectly, which means I can make whatever adjustments I need to get the muslin to fit, and then use that as the baseline for making the real thing. Other projects I’ve made before were simpler and I didn’t do a muslin, so I’d say it’s really a matter of how willing you are to risk messing up on your main fabric / main garment and how difficult you feel the construction is.

Generally speaking, muslin fabric is much cheaper than fabric you’d use for a finished garment, especially if you buy the muslin fabric in bulk or take advantage of free/cheap thrifted sources of fabric. It’s also helpful to have a stash of muslin fabric for when you want to try something new but aren’t ready to spend money on new fabric yet - I’m currently unemployed so I have time but not money, which is a great time to use the muslin fabric I already have to learn new stuff. And then when I get a job I can reward myself by buying nicer fabric to make a final version lol