I believed what you say were true, making certain things easier. For example, 1x31 for breeding.
However, in my case, there are times where even with GTL, I couldn't find suitable breeders (eg specific egg moves and IVs)
As for story part, I feel that more newbies face problems during first 3 gyms and some even struggles to beat Elite Four.
GTL provides a platform for players to buy
-berries (EV reducing berries) when they do not plant or have access to the farm (only Hoenn and Unova can plant berries)
-berry products- pp up and etc
-TMs, evolutionary stones- could be cheaper than NPC
-and also sell when u are offline
We know all these can be done without GTL, but I do not wish to go back to days where u have to be actively spamming trade channel, and be available for trade(says on Gym run) to make a small profit.
Using GTL is not a must, it's a sweet add-on feature which comes in handy