I've stained wood (USA) for decades and never used a pre-stain conditioner. This is the first I've heard of it. One article says:
"A wood conditioner is basically a wood finish reduced to a very thin consistency."
Sounds like a money-making product to use on very porous wood that has water-damage (rotted?) or has been repeatedly treated with solvents, leaving only weak wood fibers. I usually replace such wood.
There are also products for "rejuvenating" rotted wood:
I've used pre-stain conditioner when applying a dark oil-based stain to pine. It does seem to help even out the finish with soft woods. It is pretty useless on harder woods though, and since I don't typically use pine or stain I have maybe used it once or twice.
"A wood conditioner is basically a wood finish reduced to a very thin consistency."
Sounds like a money-making product to use on very porous wood that has water-damage (rotted?) or has been repeatedly treated with solvents, leaving only weak wood fibers. I usually replace such wood.
There are also products for "rejuvenating" rotted wood:
https://duckduckgo.com/?q=rejuvenate+rotted+wood&t=opera&ia=...
I suppose these have their place but I have been lucky enough to not need them.