The above explanation is very good, but in case it helps, here are my experiences, mostly based on Mesa Boogie amps I have had, and then seeing the Fractal Models react the same.
To me, I think of it like this:
Different amplifiers react differently to input signals. For example a dual rectifier when sent a full input signal with no cuts can sound flubby, loose, and undefined in the low end. sometimes this is desired for slower massive chords. Other times it is not desired, so a lot of people put a boost like a tube screamer in front, the big thing about the tube screamer is it cuts a lot of lows and boosts a lot of mids, and the amp reacts differently to this. It tends to be much tighter in the low end and allows more articulate playing to come through. The Pre EQ in the amp block can cut those lows to have a similar effect without the additional coloring of a tube screamer or other overdrive. It can also carve out irritating frequencies that your guitar produces. Depending on how strong a signal is in the low/mids/highs, it will hit the "tubes" and distort differently.
The post EQ in the amp block can shape the tone after the preamp. this is really popular in the Mesa Mark series amps with the graphic EQ. Many times you will basically cut the bass by turning the bass knob to 0 on the amp, because it causes the amp to lose its articulation. Then you can boost it after the preamp with the post EQ/Graphic EQ in the amp block. You can have a big, full sound that is still tight because you cut the unwanted frequencies that cause all that flub and muddiness, and boosted the bass after the preamp section (the typical V shape is often used, but not always the best answer so use your ears!)
The cab block has low and high cuts, and I find this handy so you can get rid of subharmonics and things like that the can make the mix sound muddy. its often helpful to cut around 70-120Hz here to clean up the sound more. You can also cut the high end off to get rid of some fizzyness, most guitar loudspeakers don't produce much meaningful sound above 7kHz, and I have found it pretty good to cut the highs at 9-12kHz in the cab block. But as always, use your ears! If you were playing live with a sound engineer mixing, he would likely cut the guitar signal similarly to this, get rid of the lows to make room for the kick drum and bass guitar, and get rid of the highs to let the cymbals shine up there.