Where else? OK there's the odd train to Dunkerque or Frethun...
I mean, from an infrastructure point of view, the French LGV/HSL ends at Fréthun. (Amusingly, the official name of that section of line is “Ligne de Frétin à Fréthun”, two names that are pronounced the same or only slightly differently depending on where your French accent is from. But I digress.)
But this device was used for the despatch of trains, so it makes sense indeed to use the major destination down the lines.
Today, domestic northbound trains at Lille Europe are not that odd compared to Eurostars - something like 10 tpd on a weekday - but the situation was quite different ~30 years ago when Lille Europe was inaugurated.