Yes it will help, I’ve been doing spin twice a week, plus using the turbo trainer through winter & this year my fitness has stayed pretty much where it was before the grotty weather started, if anything I’ve improved having done just shy of 50 miles a couple of weeks back, with about 1300 meters of climbing, but I do like to give it some welly at spin, yes the exercises are a bit silly, but the squatting one hurts your legs which is good practice for climbing when they hurt, you can practice ignoring the pain which is good, as Jensie said shut up legs and do what I tell you to do! you’ll probably find your better at spin then those who have been going for ages