I believe the online diary is more useful because it organises my daily tasks according to their importance, and because I’m alerted when each task occurs.

Main clause, adverbial clause, and adverbial clause.

The above is parrallel, but would it be better to change the second adverb clause so that the verb construction is in active voice like the first? it sounds better, and I’ve read this is considered better stylistically:

I believe the online diary is more useful because it organises my daily tasks according to their importance, and because it alerts me when each task occurs.

Also, when do you HAVE TO repeat the word that begins the parallel structure? In this sentence it is ‘because’ and Idon’t have to include it for the second clause.

Thanks.

It is best to say:

I believe the online diary is more useful because it organises my daily tasks according to their importance and alerts me when each task occurs.

You don’t need the second ‘because’, as the sentence is quite clear without it.

I seriously doubt that your diary is that smart.  The best that it can do is rank your daily tasks according to their assigned importance, and sound an alert when they are due to occur.

The diary cannot determine importance, nor can it know if the task was carried out at the time it was due to occur.