That was quite normal for the time though. Tours in those days were to promote albums. Tours would barely break even, or at most make a very small profit (this has been said about ABBA’s). The increased album sales were therefore the payback.
Obviously now the exact opposite is the case... albums don’t sell and they exist purely to promote tours (which are lucrative, hence the high ticket prices).
The pattern wasn’t like that completely though. Arrival was a huge seller (in the UK, I think their best-selling studio album) but its initial release didn’t follow a tour. Its sales were helped by the 1977 tour though. This doesn’t appear to be the case with Voulez-Vous, however, even though many of the songs on that album were performed at the 1979 concerts. Record sales were split between that album and Greatest Hits Vol 2. Perhaps the release of the latter hampered Voulez-Vous’s prolonged success.
Even as I pressed "Create Post" I thought "the exception is Arrival". I'm not surprised it was spotted! You're perfectly right.
But I'm guessing the reason the Arrival album had sold so well already, even before they'd ever toured, was simply down to the sheer explosion of interest in ABBA, mostly due to their novelty and the amazing discovery by the general public, only a year before, that they had produced many, many infectiously melodic songs - as evidenced on their Greatest Hits album of 1975/76, a massive seller practically overflowing with catchy pop songs and wonderful, lush sounds.
ABBA had all the following things in their favour: They were new, or at least they'd been newly re-discovered (and what hide, what infernal cheek!! this was a group that was bucking a revered trend of Eurovision one-hit wonders from non-English countries that disappeared forever! how dare they! and isn't that really quite amazing?!); they were photogenic and striking to look at; they had original-looking videos airing on tv; they had two extremely attractive singers with clear, vibrant voices that jumped out of radio speakers to almost stop traffic; and their recording techniques and production were impeccable.
I think this explosion of interest in ABBA, off the back of their Greatest Hits album, and with the amazing SOS single closely followed by the chart-toppers Mamma Mia and Fernando, helped create the massive anticipation for Arrival, which was basically their first studio album as established, known, superstars with a track record already. There was enormous curiosity in, and excitement for, any new stuff this suddenly huge group had just produced.
And as you say, their 1977 tour kept Arrival in the top 10 best-selling albums for many months afterwards, further bolstering its already big sales, even catapulting it once again into the number one position.