The Champions League grew out of the European Cup, founded in 1955. As assistant coach to Franz Beckenbauer at France's Olympique Marseille, he made it to the final of the Champions Cup. Of course, the prestige factor as well is pretty high. The European competitions are highly lucrative. Leeds manager David O'Leary was on track to win some of that back.
Clubs can afford to pay that kind of premium.
That has changed the way managers build their squads. No longer can a team operate with 11 starters and a few reserves.