In the manual on the chart, for 19-25 it just says "All."

What is that supposed to mean? Just that you can't get any more by raising your intelligence? If that's the case, all it does after 19 is increase your Lore...

Basically. For dialogue purposes, there's also no reason to raise int above 21.

There's a limit to the number of spells you can learn for each spell level. With int 19 or higher you can learn all the spells you can find.

-edit-
Planescape:Torment uses the 2nd edition rules. The 2nd edition rules only give bonus spells to priests for having high wisdom. This increases the number of they can memorize per day. Mages don't get more spells per day under 2nd edition rules regardless of their stats. (In 3rd edition all casters gain bonus spells per day from their stats) In Planescape: Torment mages can get extra spells per day from items though.