Essentially they were converting the euro coupons which they got for free to cash with the expected value of a bet on the roulette wheel (which is slightly negative giving the house an edge over time). For example, if the avergage expected value of the bet is -5% (the house edge), then for a single bet, on average, you would expect to get 95% back of your original bet amount. Since they got 10 euros each time for free they would expect to get back 9.5 euros on average over all their bets, given this house edge (which I made up).