Usually my riding tapers off dramatically in the winter. I get out for a ride maybe once a week, do a little running, and generally get pretty lazy. Come spring I usually need a month or so to get into it and build my fitness back up.
This season has been different. I bought a smart trainer, set my bike up in the basement, and found an app I like. I've ridden more the past two months than ever before in winter. Not only does the trainer allow me to continue riding on cold and dark days, it also gives me very effective workouts (more focused and strenuous than most outside rides). I'm looking forward to spring when I can get back out and see what a difference this has made in my general fitness. The proof will come when my favorite weekly group ride starts back up. Hopefully I will have an easier time hanging with them from the first week. I expect this will be the case, as my endurance is already improving, according to the app.
The takeaway is this: indoor trainer = huge benefit, in my experience at least.