The only answer to your question is..... when an offset ratcheting wrench won't fit.
I have both types plus a flex head set and even though I'm just a DIYer I have had
the need for a 0 degree offset spanner on more than one occasion. If this is for your
fist set of ratcheting spanners I would advocate getting a set of flex heads first as they
can fit into all places the other two types can and still do things the other two types can't.
Then you could add the other sets later if you wanted.