For cleaning houses, I have never liked the idea of any type of wet & dry machine as their bulk seemed to be unnecessary for housekeeping tasks. Compared to the Shop-Vac cleaners sold in the UK (they were branded Aqua Vac here), the quality of the Vax was far, far superior, but then they could be as much as double the price of an Aqua Vac, so one could expect the extra quality really. Vax was also a good deal quieter as Aqua Vac cleaners were always as loud as loud could be. I always thought wet & dry cleaners were more suited for use outside of the home, though for what Vax cost back in the day, I wouldn't have wanted to spend that amount of money on a cleaner to use outside.
However, for the UK, it was all about the idea that Vax did it all - even though very few consumers would have washed their carpets with it as a regular thing. Wet & dry vacuum cleaners were never all that popular here as the need to suck up water was minimal. So in effect if one bought an Aqua Vac, it would be mainly for dry vacuuming and would not be a practical choice for the home. Vax changed turned this idea on it's head as it offered even more; it sucked water and it washed carpets too. Had it been sold as a wet & dry cleaner only, I doubt many consumers would have purchased it. Likewise if it had been a carpet cleaner only. It's success lay in the fact that it did something everyone needed (a dry vacuum cleaner) as well as something which consumers were prepared to pay extra for.
Vax was very good for my business, because a good deal of upright vacuum cleaner owners replaced their upright cleaner with a canister Vax. Many of these people quickly realized they preferred an upright cleaner for dry vacuuming. However, such was the price of the Vax, buying yet another cleaner was extremely decadent, and I lost count of the times an old upright cleaner was brought in for repair on the basis that although it's owner actually wanted a new one, the Vax canister which had replaced it had proven to be a poor and expensive choice. Likewise I sold many a reasonably inexpensive reconditioned Hoover Junior or Electrolux upright to someone who'd recently bought a Vax canister.