Why Autocad has "None" option in "UNDO" command?

[MTGH9UU]

Why Autocad has "None" option in "UNDO" command? Why we should /OR/ we need to disable UNDO and U commands?
And why has "One" option for undoing only 1 operation? When can i use this options?!

[Tom Beauford]

While I would avoid disabling UNDO there could be times when doing large resource using operations that managing UNDO file could overwhelm a PC. Using the Begin & End options allows commands to be grouped as a procedure so we can reverse the procedure with a single undo. To UNDO a single command requires using the One option in this case.

[cadtag]

to expand a bit on Tom's reply --- AutoCAD _used_ to be quite usable on a 80386 25Mhz machine, with a 100Mb hard drive.

An undo file that tracked an entire day's worth of edits for a single dwg could potentially overwhelm the small hard drives that were available. setting undo to None was occasionally necessary to avoid filling up the hard drive. Setting it to One still gave users a limited Undo capability without the risk of crashing the system after filling the hard drive.

Today.... with Terabyte SSD drives around, neither the None or One settings are that useful or necessary.

[MTGH9UU]

Good Answers!
Thank you very much, @Tom Beauford, @cadtag