Reasons a person might want to use an expandable even if their bow is well tuned.
1. Much like a field tip and expandable head is less affected by the wind.
2. Fixed blade heads are like poor shooting form amplifiers. Not much of a problem with shooting form standing on flat ground in the front yard. But 30 foot up a tree; leaning out around a limb, wearing a thick coat in freezing cold weather is another story. Mech heads are as forgiving in such circumstances as a field tip.
3. Lets say you have a 30 inch draw length and are shooting a 70 lb bow generating massive amounts of kinetic energy. Some people like that figure, "why waste all that kinetic energy driving a fixed head another three inches deeper in the ground behind a deer (which is of no extra advantage) when I could be using it to make a bigger hole in the deer". Ever try to tune a 1 3/4 or 2 inch fixed head to shoot well at 300 fps? That's where an expandable really shines. Flys like a field tip but cuts a hole way bigger than any fixed head you could get to fly well out of todays fast bows.
That said, I use both fixed and expandables. Both have their advantages in certain circumstances.
Random observations:
Don't use a mech head to cover for an untuned bow. Tune your bow!!!
Don't use a real wide diameter expandable if your bow doesn't have the power to push it all the way through a deer. Know this by actually shooting your bow through a chronograph and then plugging that speed and the weight of your arrow into the formula for kinetic energy.
The vast, vast, vast, majority of the "It didn't open stories" about expandables are horse ####.
I could give you a list of expandables that penetrate just as well as most fixed heads.