The talent grants you +3 to specific attribute checks (e.g. if you take the Improved dexterity ability, you will get +3 when sneaking, searching for traps, scaling sheer surfaces).
If you improve the DEX attribute, you get multiple benefits:
+1 to attribute checks (so similarly sneaking, searching for traps, scaling sheer surfaces)
+1 to saving throws (breath, mechanical traps - mind, there is a talent that grants +3 to these too, Improved dexterity saves)
+1 to Def
+1 to ranged weapon attacks
In other words, the attribute increases a lot of different stats, while the talent allows you focused increase in a specific area.