Ou and Ou bien mean exactly the same thing, and are switchable.

In this text, having just a Ou, at the beginning of a sentence doesn’t seem that nice to hear/read; thus it was replaced by its exact equivalent.

It seems to me the main reason why you should use “ou bien” in general is because “ou” is ambiguous: it may be “où”. Look at the following sentences:

Je vais chanter ou je vais danser.

Je vais chanter où je vais danser.

You cannot decide between them when you hear them.
If you say “Je vais chanter ou bien je vais danser.”, it is not ambiguous anymore. You can also say “Je vais chanter ou danser.”, as there is no more ambiguity there ; obviously, it is still possible to say “Je vais chanter ou bien danser”, but then you are stressing the “ou bien”.

For this reason, “ou bien” is often used instead of “ou” in colloquial french to link propositions, which is exactly the case in your example.

This is to add to the existing answers, since I think there could be more than one reason.

Keep in mind that « ou bien » is stronger than simply « ou », in the same way that « bien » often has no function except to emphasize. « Ou » is a hard word to put emphasis on, particularly in writing, so it benefits from the support here.

Depending on the context, you might translate it as more than just “or” — for example “or even”, “or else”, “otherwise”, “on the other hand”.

Or you might even (ou bien) choose some way of contrasting with an earlier element. For example, here you have the earlier element « parfois », which could be countered by « ou bien » :

Sometimes we need supplies for dinner on Saturday night. Other times the holidays are coming up…

(This is a bit freer in terms of word-for-word translation, but I think it works better than just “or”.)

"Ou" means ‘or’, but "ou bien" means ‘or else’.