Rito we have a word for 99% slows.

Slows don't prevent the use of abilities that involve a dash/blink, certain abilities (Garen Q, etc.) interact with slows but not roots, etc.

No, a 99% slow doesn't actually reduce your ms by 99%, but is guaranteed to put you at the minimum of 110 MS, ehereas a root holds you in place for some duration

A root silences dash and blink abilities, a slow does not

A slow can be cleansed by Garen Q, Yi ult, Blue Kayn's E, a root cannot

etc

1 - Movement speed can only be reduced to a certain amount, a 99 percent slow is in fact not a 99 percent slow.

2 - Slows can be reduced or cleansed entirely through slow resistance/tenacity, abilities (such as Garen's W or Master Yi's R), items (QSS, Mercurial Scimitar, and Mikael's Crucible), and the summoner spell Cleanse.

3 - You can dash, blink, and use other movement commands when slowed.

4 - Slows decay over time, roots do not.