Chain tension and tensioning a chain

Nobody actually needs a chain tensioners/tugs. Some people find it easier with them, some don't feel the need to use them.

Mine are semi vertical.

I suppose what I'm wondering is, when Sheldon says 'These are a problem when you want to dispense with a derailer, because you need some way to regulate chain tension.' why vertical dropouts don't guarantee chain tension but horizontal ones do.

thanks