Bicycle Mechanics - Why use sew ups for boot and not clincher?

I've been reading threads about booting a tire because I have a 75 mile ride tomorrow and have to do it on a booted tire... All the threads I read said to use a section of sew up tire as a boot. I don't have tubulars and never have. I've never even seen one in person. I'm using a piece of clincher that I glued into place and tried to feather the edges so they won't wear a hole in my tube. I'm just wondering what the difference between a sew up and clincher is that makes a sew up better to use.
Probably a dumb question. Sorry.

you can peel the tread off a good (cold processed) sewup, then peel off the base tape, then you cut the stiches sewing the tire together, and cut off double thickness of the tire body that the stiches went through. You are left with a soft, thin boot, that works better than anything else for booting a clincher. You do this on a 6 inch piece of tire, then cut into 5 or 6 boots

you could just cut up some denim pants to make boot patches too.

Thanks for the detailed explanation. I managed to peel the thread off the clincher using a flat head screwdriver and pliers. It held up today. I forgot my LBS is closed today and tomorrow, so I'll be doing another 50 miles on it tomorrow.. I'll carry pieces of demin from now on though. Having a clincher super glued to the inside of the tire makes mounting the tire really difficult, it was difficult in the first place though.

they are a thinner tread.. , I used cheap vulcanized sew-ups,
tread won't pull off those..