Champions that need to quickly escape team fights or, vice versa, chase down escaping enemy team members will greatly benefit from the tenacity/speed bonuses. Escapees will lower the risk of being pinned or killed by enemy slow or snare abilities, and chasers will be able to keep up better with slows and traps being used against them.
Slow Reduction Example
Maokai has 335 moving speed. With Mercury's Treads it is 380.
Relative Moving Speed: Normally can takes distance 1140 in 3 sec (380ms*3s). With 20% slow it is 593 (304ms*1,95s) (reduced moving speed with 20%, reduced duration with 35%) and 399 then (380ms*1,05s) -> sum 992 (593+399). 992/3sec = 331ms is the Realtiv moving speed.
Relative Slow Effect: Normal Speed (380ms) slowed with this amount (13%) to Realtive Moving speed (331ms)