Hola Nicole,

In this sentence from the exercise:

...y sufrió muchas tormentas durante su travesía. 

we are using El Indefinido (sufrió) to express that this is something that happened in the past within that period of time "during his journey". We see this action (going through storms) as something "completed". It had a beginning and an end. The use of the preterite here is not descriptive (like the Imperfecto, sufría). We are using the preterite to see this as something completed. 

We could have used the Imperfect if our aim was that of a "descriptive scene", background information,  without giving emphasis to the completion of the action. 

Here is a lesson on the general contrast between these two tenses.

Un saludo

Inma