The importance of AbilitySpecial values

To specify numeric values, you can put in a number or you can use %name formatting to grab values out of the "AbilitySpecial" block of the ability. The advantage to using the %name syntax is that the value can change as the ability levels up and the numeric value can be formatted into tooltips.

When coding abilities or items, do not fall into the trap of replacing the use of AbilitySpecial variables with a constant (i.e. writing "Duration" "12", "MODIFIER_PROPERTY_MOVESPEED_BONUS_PERCENTAGE" "-30" or doing similar assignations in lua scripts), thinking it will only be used once.

There's 2 problems with doing this

Tooltips are hard
If you don't use AbilitySpecial for each variable, when you get to making the tooltips, you'll find pretty much impossible to make quality dota-styled strings because your spell description needs to have direct copies of the values you've put in the ability. To make this worse, if you ever make a change to an static number in your datadriven ability, you will also need to update the addon_english.txt
Consistency between Lua & Key Values
Changing a key value won't only affect the datadriven but also the scripts and its easy to make a change and forget to extend this change to the .lua file. Doing proper references to the Specials also eliminates this problem.

**TL;DR**: Use as many AbilitySpecial values as possible, then modifying/balancing your abilities can be done just by changing these variables and it will extend to the rest of the game mode.