It is a personal preference. PADS has the long standing 'bug' of not supporting negative coordinates in the CAM output, so if you don't use the lower left corner you'll need to use offset in the CAM output. I got in the habit long ago of putting the lower left corner at 1000, 1000 so i could have dimensions on teh left/lower side without requiring an offset (although I typically use offset anyway for consistency between the CAM files). .
Occasionally a PCBA will need a common reference with another PCBA or mechanical fixture so it makes sense to put the origin at that point. it varies by design and designer
Using a hole, preferably a tightly toleranced tooling hole, provides a common reference between the board outline dimensions and the drill data as a feature that can be easily measured on a delivered board. Hole locations are usually more tightly toleranced than the board outline, and using a hole as the origin is the easiest way to correctly dimension them on the fab drawing. Using the lower leftmost hole is more of a convention than a requirement, since it presumes a board has a mounting hole in that location, but it is the recommended location in IPC-325. All photoplotted artwork is laid up using a tooling hole, so using a common dimension for all the data makes the most sense. The gerber files can be easily aligned to use the hole as an origin with the offset setting.
Those are the historical conventions, but with today's CAD systems, it is less critical. So when designers use the board outline as an origin, it used to be more susceptible to errors, but since CAM operators can re-align everything to what they need to fab the board, they can get the job done no matter what you use. If you want formal documentation to match IPC standards, use a tooling hole as zero. If it's an informal production with less strict documentation requirements, it doesn't matter.