Flash Gordon - Why the dialogue?

While looking for the score I found this:

Composer's note: 'I was brought in in a crisis
situation when it was found that the composer nominated by 'Queen' had
for some reason been unable to complete 'more than one minute' of a
score for the film. I was summoned to a meeting at CTS Wembley by sound
recordist John Richards and Brendan Cahill head of music for Universal
Studios Hollywood. The RPO had been booked for two weeks and started
recording the day before but had nothing else to play. I said it would
take at least 4 weeks to write the amount of music required, possibly 90
minutes. After ferocious negotiations with my agent Liz Keys at London
Management I began work, but the time gradually whittled down to 10 days
and the last 4 days of that I didn't sleep. An added complication was
to include various guitar phrases and the song 'Flash' within my
large-scale score for 80-piece orchestra. Somehow I finished it and
conducted the 3 days of recording sessions, but afterwards I went back
to my house in Mortlake and collapsed exhausted. My wife had left the
house with the 2 children at the end of my first writing day, bothered
by endless phone calls and courier bikes. She returned on the Saturday
expecting me to have left for France on the Thursday. In fact I had been
asleep for 3 days. She called a doctor who injected me with something
to wake me up. He said it was possible I would never have woken up at
all, since I was suffering from chronic bronchitis due to total
exhaustion! Anyway I recovered and I and everybody else were pleased
with the score. Dubbing sessions began and I later discovered that much
of my score had been replaced with synthesized music, myself having
demonstrated how to handle it. A disappointment. However, my
relations with the members of Queen were always cordial. Brian May came
over one day and hummed an idea for an 'overture'. As he did so I jotted
it down on some manuscript paper and then played it back on the piano,
which really startled him. They all came along to the orchestral
recordings and seemed fascinated. I remember Freddie Mercury singing the
idea of 'Ride to Arboria' in his high falsetto and I showed him how I
could expand it into the orchestral section now on the film, with which
he seemed very pleased. Whilst scoring I had cassettes of guitar ideas
from Brian, in particular the slow 'falling-chord' sequence. I wrote
this out into my score at one point and surrounded it with big
orchestral colour. When I came to the recording I had Brian's solo
guitar on headphones and conducted the orchestra in synch. around it.
Many months later Brian came over and we listened to the finished
album.'

Also - I found the score. It's brilliant. They should've found room for Howard Blake's unused version of The Hero on the bonus EP. It's amazing.

there are two kinds of OST albums that accompany films..

1. a collection of songs (either by one or several artists)
2. a collection of pieces of music

for Flash Gordon queen chose option 2. (there are two songs on the album - but the rest is really incidental/atomospheric music pieces)

it'd be difficult to sell this kind of album - OSTs don't tend to sell very well unless there are actual songs on them.

so to try and generate interest in both the film and album the dialogue was added

It's worth mentioning too that it was only a couple of years after Jeff Wayne's War Of The World - perhaps Queen envisaged something along those lines. Of course, if the film had done better it might have ended up being one of their biggest albums. They learned to not put all their eggs in one basket next time a film OST presented itself to them.