What is the purpose of this device?
It’s an early Canal+ 'pirate' box (with props to commenters who mentioned that.)

The clue is the set of delay lines. Canal+ was originally based on a scrambling scheme called 'RITC Discret 1', that randomly manipulated video delay in increments of 902ns (0, 902, 1804ns). Four 470ns delay lines (those green Philips things in the photo) would give 1880ns or 940ns in the middle, so fairly close.

More about that here: https://en.wikipedia.org/wiki/Television_encryption#RITC_Discret_1

And here it is being discussed (in French): https://www.smartphonefrance.info/actu.asp?ID=12502

And... a theory of operation (also in French): https://team-adn.eu/electronique/decodeur-canal-1984/

Translated to English, sorta.

Anyway, so the job of the HC05 is to figure out the current Discret 1 keying based on a reference that's periodically broadcast, then implement the variable delay line-by-line, selecting one of the Philips delay taps.

What happened with this box? With the publication of the design in the December 1984 Radio Plans, it was widely produced, ultimately leading Canal+ to switch to a different scheme.