[Hope]

In case this isn't obvious: You can review all items in a context that is on hold in context mode by selecting that context from the list of contexts on the left. Reviewing these contexts could be part of a weekly review.

I'm not sure that "on hold" is the "intended" function for implementing a "waiting for" context. It is one possibility that will suit some users. Personally, I use future start dates in a context that is not on hold.

On hold functionality has other uses. Gardener2 describes a way to set up dependent projects with an on hold context. I use this method, but only put the first (sequential) action in the on hold context. That way I can assign the appropriate contexts to other actions, but don't have to see them until they are actually available to me.

Contexts can be on hold for other reasons. I may have a "laminator" context, but then lend my laminator to a friend. While I don't have access to the laminator, I can set the context on hold. All of my laminating tasks will stay out of my action lists until I have my laminator back.

Or I may travel a lot. I may have some tasks that I can only do in my Toronto office, and others to do in my Edmonton office. While I'm in Toronto, I don't really need to see the Edmonton items on my action list, so I put that context on hold. Or a colleague who gets his own "Agenda" context may be on vacation. I'll put that context on hold until he's available.

OmniFocus has a lot of flexibility built in - so that you can arrange things the way you like. People may want to have different contexts or projects or even folders disappear from sight for all kinds of reasons. There are a number of ways to make this happen, including "on hold" functionality. It's there as an option - not an official implementation of any particular GTD list.

[Edit: spelling...]