Because even with my 10 years of breeding I can't feel 2mm folicals or tell the difference between 10mm and 12mm if you're want to not over work your males ultrasounds are useful tools. They allow a breeder to pair animals when it is most advantageous for the male to properly copulate at the key times in follicular development. It mean a male can breed 8-10 females without stressing him self out or wasting sperm in the process.
So yes you can feel follicles but with an ultrasound I can see them measure them accurately and overall get a much more accurate idea of where in the development a female is.
I suggest everyone learn to palpate females but an ultrasound is another tool in the bag of tricks.
Hope that helps.