The F lounges are often smaller...if they're busy/noisy/hot they become less desirable, despite the (ever-worsening, it seems) champagne and JW Blue! The LGW F is pretty small for example, and I've nipped out of it in to Terraces once recently instead which has far more space and was quieter.

Likewise, some facilities aren't available in both places, e.g. (IIRC, yet to use it as a Gold/in F) at JFK the showers are on the Terraces side.

Phil