I only submit once a year as well. The odd time I have done it twice when I get a last minute unexpected one at the end of the year.
But some people may submit more frequently, depends on how they want to manage it I suppose. No benefit really, other than just smaller amounts to deal with at a time.
For me it just works out better to stay with the percent fee. For others the flat rate may be better. To be honest, this year I expect I would come out ahead with the flat rate due to a big expense for one of my kids. But not worth paying some $150 setup fee to switch over.