I use this tool to stretch mild steel exhaust tubing, so I can do slip-fit assembly of all my bends, straights, cats and mufflers for my DIY custom exhaust. I don't use clamps - I weld - but making everything slip-fit lets me put the WHOLE system together and get it aligned perfectly, then hit the joints with a couple tack welds to maintain the alignments. Then I can remove the tubes from the car and finish weld on my table. For an amateur welder like me, lapped joints are a ton easier to get clean welds on too. Butt welds need very tight, no-gap lineup between the pipes, which means a ton of test fitting, tube end filing, and refitting. A serious time suck.
Or if I'm replacing stock/OEM exhaust parts that have worn out on one of my street cars, this is also helpful if something isn't lined up quite right and I need to make a coupler or something.
I really doubt it would stretch stainless. Maybe if it was really thin, I don't know.
The O-rings that hold the tool segments together are rubber/elastomer material, and they do break from time to time. Lisle part numbers for replacements are 34500 for the small and 34490 for the larger one.