You don’t need them , musket caps were made large for military purposes. Easy to handle under stress and such. If common caps weren’t effective no one would use them yet they are and have been in use for a couple of hundred years.
I used to use musket nipples and caps on my TC and my Traditions because I was getting inconsistent ignition, tuned out to be the Pyrodex in my particular case when I first started out. When I switched to black powder the issue went away and I switched back to regular #11 nipples, just my personal experience.