35mm films were frequently “blown up” to 70mm to take advantage of the six-track magnetic stereo soundtracks that were used for 70mm release prints. (The extra-wide gate also meant more light was being thrown onto the screen compared to a 35mm print.) The popularity of “Star Wars” and “Superman” led to many more theaters installing 70mm projectors and sound systems. But as digital sound (which could be encoded onto 35mm prints, or played in sync via CDs) came into play, the cost of 70mm prints (much more than 35mm, especially given the magnetic audio stripes) was no longer palatable.

Here is a link to a list of 70mm blow-ups.
http://in70mm.com/library/blow_up/index.htm

For example, in 1982 there were 29 films shot in 35mm that were released (or re-released) in 70mm stereo.