As such a monsterous tank you need to be everywhere, telport allows that and lets you quickly grab an item and then going back to feast on some minions.
Some Other Great Summoner Spells

Ghost and/or Flash: good for maintaing feast stacks whether you need to run away or catch up to eat them they are both great for cho gath.

Exhaust: Great for getting first blood, killing a annoying dps champion with madreds blood razor or just slowing someone down so they can get Feasted.

Ignite: I normally dont like ignite for Cho Gath but the reduced healing does come in handy when you need someone to die, along with some damage over time it makes it easy to get off a Feast.

Heal or Revive?: Although most people think these summoner spells are only good for noobs they all have a time and a place. Even though revive has a very long cooldown it could be very usefull becuase as a tank Cho Gaths presence is always required. Heal is a good spell for Cho Gath it could keep a push going or even save a team mate or preserve feast stacks.

Clarity: If you have problems farming on Cho'gath and maintaining mana then this is a good option, if you are comfortable sustaining your mana pool then this spell is worthless, only take it if you know that not taking it would ruin you.

Cleanse: This is a great spell that is just not used enough, I would only use this spell if I replaced mecury treads on Cho Gath but it can save your life or help you get a kill. Avoid this spell unless there is HEAVY crowd control.


Bad Spells For Cho Gath

Rally: Cho Gath does not need the extra help in a team fight due to the large amount of Crowd Control he has. Even if you do want this spell its only real purpose would be to push a tower.

Clairvoyance: Since Cho Gath is relitivaly slow and short ranged this spell would be pretty much useless.

Fortify: This can be a great spell, making your tower invincible and giving it crazy attacks speed. In my oppinion Cho Gath does not need this becuase he needs other spells much more.