This pattern is used to talk about hypothetical situations that did not actually happen, and what the result would have been. The たら-clause sets up the condition, and ところだ (often in the past form ところだった) describes the state or point you would be at now if that condition had been true. It emphasizes that reality is different from that hypothetical state.

It is often used with expressions like もう少しで (almost) or 危うく (narrowly) to say that something almost happened but didn't. For example, if you almost missed a train, you might say もう少しで乗り遅れるところだった (I was about to miss the train). But with たら〜ところだ, it's more about a complete counterfactual: if X had happened, then Y would be the case now. For instance, もし彼が来なかったら、今ごろは独りでいるところだ (If he hadn't come, I would be alone right now).

Note that this pattern is quite formal and literary. In everyday conversation, you would often say 〜たら〜のに or just use 〜ていたら〜ていた. But in written or dramatic contexts, たら〜ところだ adds a strong sense of 'would be in that state right now'.