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My guess would be it's the "safe" option especially if avoidance calculation isn't actually being done (also may not even work well with diff printer configs, multiple models). And yeah, slow Z travel.
Though I haven't done 3D printing in a while (I should have kept it simple) so I don't know. I would probably try to use some other print as a purge if possible, either via infill or just something that doesn't matter (utility or will be painted anyway).
It's so you can't wind up crashing anything into the print. There is only a finite amount you can move the print head back towards the plate (or the plate back towards the head; however your printer works) before you run the risk of hitting the print with some mechanical component. Be it the X/Y axis gantry, the housing around the print head, or anything else. Sure, you could theoretically tune this maximum value to be whatever your particular combination of width/depth/height of toolhead enclosure is, and its offset from the gantry, etc., etc. but 100% effective avoidance pathing is difficult and, in the event your print bed is mostly filled, potentially impossible.
So it's safest to just not do that. It can always be assumed the nozzle can move around at or one layer above the current print height without hitting anything. Below that level is increasingly risky.
Understood. Not a factor in what I have been printing so far, but I can see it for the generic case.
Maybe a flag could be nice, but eh. It's fine.
I usually just disable it. Didn't see any noticable differences with or without it.
The only case where I enabled it just in case was when printing with PETG and using PLA as supports.
As to why it has to be as high: I think the printer is not so smart. Depending on the tower and object position it might crash the X axis gantry into your print if it had to go down for the prime tower. (I assume that because there are lots of unnecessary safety margins like this when printing multiple objects one by one)
There’s also the retraction/wipe of filament. So if you can’t do it inside the print then the tower is used to give that surface to wipe off against.
Worst case then would be boogers/wisps
To be honest I haven't tried without it (didn't want to risk the rest of the filament if it would turn out poorly), but I reduce the size to 15mm/15mm or so, plus flushing into infill/supports, and has been ok so far.
I'll try without it sometime when I print for my kid and don't care as much :)