The companies selling the seals will tell you it increases conversions.
We have have had many, many discussions here at WebmasterWorld about them (do a site search). Some people claim they work. Many years ago I was very sceptical about the whole thing. But over the last few years I have seen very large ecom retailers using them. It could be that they are just hopeful or are buying the sales pitch. But it could also be they have done A/B testing and have conclusive evidence.
It really shocks me that they are charging so much money for the seal. I cant believe it is worth it. But I guess if you are doing 10's of millions in sales increasing the conversion rate by a faction of a percent makes it worth it.