Because if you follow the instructions to the letter, it definitly *will* work. On the other hand, there have been so many cases where people didn't bother to read rather follow the instructions and threw everything anywhere into the game folder, then came here and wondered why nothing worked. I also had some guys actually putting the mod into the folder *twice*, not just into the wrong place. Then there were those people who only reluctantly after servere questioning admitted that they used outdated mods together with this mod. Or outdated game versions.
Clean install is required sometimes when between mod versions one or more files were deleted. If they remain in the game folder, this is a source for strange errors. Of course, it is possible to maintain everything manually as it has to be, but doing so will definitly provoke errors.
The game itself as it is has some issues with save compatibility, and the official recommendation is to save only in the main town map or MC's bedroom. Saving while on other places does not *have* to cause errors, but it had in the past, so better safe than sorry. A saved game contains a pickled representation of any Ren'Py object in the store that is active at the time, and if one of those object was subject to change by the mod, it might become a problem. Also, if you save while in the middle of a quest or a dialog, and the very same quest or dialog was changed by the mod (or a new version of mod or game), you are in an undefined state and most likely get logical and even game breaking errors, even though the saves appear to be compatible and working. Such risks are minimized (not eliminated -- if you really want you can still break the game!) if you only save in those places.
This is also the reason why I put so many cross-checks into the mod which verify matching mod and game versions, the folder where the main mod files are located, and so on. Therefore, if you put the mod into the wrong folder, or a non-matching version, it will recognize and complain itself, if the game is still able to start. Since the OP simply did not get any message I strongly suspect that he did not install the mod at all.