They may be looking for information that can help them improve the claims. If you can tell them how you might detect use of your invention, then they can try and claim some of those markers, or at least disclose them.
For example, "a new and useful way of making widgets" may or may not be easy to detect. But if can determine that someone is using your new and useful way because doing so makes the widget shinier than traditionally made widgets, because it reduces production time, because it leads to consumption of fewer resources, or because it causes the widgets to come off the assembly line in unusual orientations, then they can add some or all of those elements to the claim or disclose them in the patent.
You might not have thought of any of those things as being part of your invention, but there can be value in having some claims or disclosures that mention these things.
They may also be making the point that unless you have a way of detecting infringement, you might want to think hard about spending the money on the patent.
These are just guesses. What you should really do is ask you attorneys.