Tryndamere is known to be a rather easy, and basic champion. He has fast/efficient jungle and waveclear once he completes shiv, scales super hard into lategame (Especially with this build). His damage is absurd, but is needed to be that way to make up for his lack of hard CC or range. This idea behind this build is to make it so you constantly and consistently have a way to keep on the target, while still having the damage to win the fights regardless.

Cooldown reduction on tryndamere, done in this manner is all for the purpose of keeping on the target with ease. The stormraiders is taken partially for the bonus % CDR and the increased cap on CDR, and for the pure movement speed boost, the slow reduction makes him difficult to escape, and i quite often end up getting kills in lane because of this mastery, or get a laner to need to blow summoners and everything just to escape, and quite often you can end up basically outrunning someone's flash with this mastery and keep on them.

The big appeal, and why cooldown actually gives SO MUCH more mobility, is because of the passive effect on his E. Having 45% cooldown on tryndamere will put his E on a 4.95 second cooldown, and note that this cooldown is reduced by 2 seconds every time he crits on an enemy champion. Having his rage bar up, along with shiv and ie will put tryn at an 85% critical strike chance. So this all being said, if you E onto someone, auto attack them twice, if both critically strike, then his E will be off cooldown already, and he can spin again, between this constantly spammable gap closer, the speed buff on stormraiders surge, and having flash as another backup, there is basically no way that they can escape when you are chasing someone down. This also makes it easy enough to charge into teamfights, stay on and assassinate the enemy carry during your ultimate, without the worry of them getting away due to a flash or something of the sort.