What does this solve?

Apparently it changes nothing, but makes the pathfinder quit earlier if it goes past the max_penalty.

Seems that the penalty is only used for automatic servicing. Manual sending to depot still uses unlimited penalty.

@SamuXarick Yes, it leaves earlier and the max_distance penalty is used. For consistency, either max_distance should be used or removed from the code. I forgot to add a patch for it, but I wrote them two years ago (YAPF and NPF):
J0anJosep@d6a311b
J0anJosep@3680d95
Sorry for forgetting about it. Double work done here.

Anyway, I don't see why a change in max_distance is needed.

Because

Oh, I did that for NPF too, included it in my other patch request #6928

e556519#diff-fe68d910d6c0b0a8d81eb65e88e60fb1

Because

OpenTTD/src/pathfinder/yapf/yapf.h

Line 70 in 25a060b
FindDepotData YapfRoadVehicleFindNearestDepot(const RoadVehicle *v, int max_penalty);

Oh, I did that for NPF too, included it in my other patch request #6928

e556519#diff-fe68d910d6c0b0a8d81eb65e88e60fb1

In the file you referenced, both max_distance and max_penalty are used and their meaning is the same. It is explained in the comments. If it is an inconsistency, it shouldn't be dealt here but in a separate commit.

We recently switched from Jenkins as CI to Azure Pipelines as CI. This means you need to rebase before this Pull Request will pass its checks. Sorry for the troubles!

Same issue is addressed by #7009, which is merged.