actually, INT, WIS and CHA are all responsible for dialogue options, sazerac's INT example is the one that first sprung to my mind too, although there are other examples e.g. in the bard stronghold where
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an INT of 19 is required to re-write the musical score for the production whilst maintaining a good standard of work. WIS helps when dealing with the beholder in the Sahuagin city, and CHA can basically help you solve quests e.g. in Brynnlaw, you can gain entry into the asylum through the pirate lord dialog if you CHA is high enough. These are just a few examples, there are many more situations where these stats help dialog.
INT, as stated above denotes the number of spells of each spell level that can be scribed into the spell book. It doesn't affect how many spells can be cast though, as this is determined by the level of the wizard. It also provides a bonus to lore which helps with the identifying of items.