Heimerdinger's constant lane harass, stun/slow with [E], and Rylai's, allows him to consistently land Comet for increase lane pressure and kill potential.
Manaflow Band's early game mana sustain is perfect for empowering your early laning phase harass to push your enemy out of lane and keep the pressure up.
Transcendence puts you at CDR cap in the mid-game with just Morello's and Zhonya's enabling you to itemize into Magic Penetration early.
Scorch empowers your ability to abuse your enemy laner in the early to mid game and press your lead. Heimer, like Teemo, is all about applying early pressure and not letting up.
Zhonya's is a key ingredient to Heimerdinger's arsenal of items and Perfect timing allows you to hit that item power spike before your enemy knows what hit 'em.
The early stasis also provides you the opportunity to potentially out-play an enemy jungle gank in a 1v2, if timed well.
More CDR means more lane harass and more pressure on the enemy. The extra little bit of Summoner CDR also means when the enemy matches your Teleport top you'll have the advantage when it comes back up.