I first encountered this wonderful invention when staying with a friend in France about 10 years ago. I don't know if it's regional or simply a French thing, but it is a very simple idea.
Take your beer - preferably a continental one - Kronenbourg is ideal, pour into your typical French beer glass and add some "sirop". Sirop is like our cordial really (Robinsons etc...) although the French version does seem to be somewhat sweeter, richer and more refined. In which case you may have to sample this on your next trip to France - I don't think Beer and Lemon Barley water would be quite as good somehow....I gained a particular appreciation for the Strawberry version, but also found Kronenbourg plus Peach to be very pleasing.
I guess this is something akin to the idea of "Lager and Lime". Is it for those who find the taste of beer way too bitter? Possibly. Mixing your premium lager with lime cordial is certainly frowned upon in some public houses, but I always think the idea of drinking a "Demi-Fraise" in France, outside a bar in a little village somewhere to be quite a nice idea. And it does taste good. The perfect refreshing drink for a warm day in the sun with the added bonus of making you "alcohol-happy"!
Just thought I would share!