Yes, but only if you approach an indoor trainer as a fundamental part of your training routine. If you just add indoor rides as an occasional thing with ad-hoc workouts, it probably will not have as much of an impact.

You can build hill strength on flat outdoor rides, but it takes a crapton of discipline. On the other hand, you can do structured workouts on a trainer and get a boatload more benefit than outdoor rides in a shorter amount of time. I ride almost exclusively inside, and I have been well-prepared for a number of mountain rides with 5K - 11K feet of climbing.

since January ive always road outside 2 reasons, 1. didn't have a trainer 2. cause I felt if it rains, traffic, whatever I cant get that with a trainer. so next year I will do a IM with HILLS and am trying to get the most hill work without having to drive 4 hours (one way) every weekend to get some. where can I find some of these workouts so I can incorporate them once a week into my ride regime? thanks

I would imagine it depends on the type of trainer too. A general fluid trainer, while certainly capable of doing a good workout, might not be as accurate for simulating hill climb workouts as a smart trainer in that you can get more resistance at lower rpm with the smart trainer. In other words, to get a high power, low rpm workout with a fluid trainer you may need more than a 50-11 setup on your bike.

However, I agree that structured workouts on a trainer can be more beneficial than general rides outdoors.

im training solo, finding programs on the web, still learning the lengo etc. where can I find "structured" workouts for hills on a trainer? any help is appreciated, thanks

I don't know about specific hill workouts, but you should be considering doing intervals that are about the same duration as the hills you want to climb. The consideration is that you want to do some of these intervals at a low cadence to simulate climbing.

Lots of workouts available here to look at and choose from: https://whatsonzwift.com/workouts/

It is not so much about one magic workout, but the mix of workouts over time that build you up for the hills. For example, I follow Trainerroad's plans from end to end, and when I have the opportunity to ride outside, I substitute that for a TR workout. TR's "Sustained Power Build" plan has a mix of extended intervals near your thresholds to develop your ability to handle longer, harder climbs rather than typical rollers. It is not something that would probably work as well with just one workout/week. But, if you were looking for "the one," then I would look for workouts with long intervals (several ~20 minutes) at around 90% FTP. Or, the other good one is a bunch of shorter (6 - 8 minute) intervals at 108% FTP.

You do not need Trainerroad if you only want one workout/week. Most of the trainers come with training apps that have workouts like these. The key is finding those extended intervals that make you want to cry.