There’s a huge difference between distortion obtained in a natural way from overdriving a tube amplifier, and distortion gained through digital modeling or simulated patches. One is alive, responsive and warm, while the other is generic, bland and cold. Despite huge developments in sound modeling, post production editing and digital effects, there remains a tangible gap between analog and digital tone, and this can be seen mainly through distortion effects.
Tubes are not only useful for allowing a sound to be amplified when used in the context of a guitar amplifier, but they are also extremely useful for crafting a certain tone, rich in harmonics and warmth. Tube tone not only sounds great, but it is also incredibly responsive to your different playing styles, which gives tube amps an organic, personal touch and allows them to work with the guitarist on a whole other level. When you drive the tubes into overdrive, this is where they really shine out. Natural clipping occurs, which breaks up the guitar signal into a wonderful, choppy crunch tone that is the foundation of every classic rock song since Chuck Berry.
Digital distortion, on the other hand, is not quite so fantastic. For a start, distortion was originally created with tubes, and so any kind of digital distortion is really just trying to use modern technology to emulate the tone. As is the case with pretty much anything which is copied, it is possible to get extremely close to the original, but impossible to ever get 100%. For this reason, there will never really be a digital distortion which sounds quite right. Digital distortions, while they do offer some benefits like pre-programmable patches and storage banks for easily recalled settings during a set and high gain models with very little unwanted background noise, are just not as responsive as analog distortions can be, and so the experience of playing, as well as the quality of the distortion tone, will always get in the way of a truly enjoyable playing experience.