One major chance to use Object Database is when you need inter-process communication. You have options. File? Socket?

How do you manage an identity of an object? Is it possible to lock an object?

Isn't it a good reason to try Object Database?

Another is when you running out of memory spaces. Suppose you work on A.I. prototype that can take pasts into account, as time goes by, you're in the situation.

It's the best if all the objects fit in memory. But it costs high, both in hardware and power consumption. So it is natural to swap some objects off to disk. The whole graph is not always required. Some are required, others are preferred. Depending on the preference, stays in memory(cached) or stored on disk. But still, in logical representation, your objects exist, consist of the whole graph. Nice, eh?

If your situation were the mix of those two, it is highly recommended to try Object Database. Thus, Object Database can be considered to extend your OO programming language.