Earlier quoted context omitted.
Only front loaders have it. Top loaders don't spin fast enough to need it.
I was of the understanding top loaders don't require it because the axis of spin is oriented such that, so long as the load is sufficiently balanced, it doesn't matter how fast it spins. Front loaders have a harder time balancing the load due to the orientation of the drum, so require greater inertial mass to stabilise them.
No, it's simply that top loaders just don't spin very fast.
> Front loaders have a harder time balancing the load due to the orientation of the drum
The opposite - it's easier for a front loader to balance the load, not harder (that's why they are able to spin faster).
A top loader has no ability to move the clothing into balance. But a front loader can - it spins slowly so that the clothing moves around, and then when it detects balance, it speeds up and locks the clothing into position.
A front loader is a better washing machine in every single way, except for one: it needs to be water tight, which means air tight, so it can grow mildew if the door is not left open when it's not in use.