You're never going to have your GTI/GTD worth less than GFV unless something catastrophic happens financially with the car market or you've heavily abused/neglected it in the time you had it.
VW are still by far a private sales car company and don't dabble heavily with fleet sales, which helps residuals, unlike Vauxhall, Ford etc who see their cars dumped on the auction market wholesale when the fleets and rental companies have finished with them.
Even if you aren't financing through solutions, it is handy to know what the GFV is for the expected mileage, had you financed that way. It's a worst case scenario value for a car in good average condition, you can add a grand to your GFV on a GTD/GTD for real value at 3 years old/average mileage if it has been looked after, they like to make you think you have some equity in the car when you go back for your next car - it encourages brand loyalty and repeat business for another new car, rather than holding onto the one you've got.