I don't get it, doesn't this only proof that the picture exists on the specific time?
You can match a coin with it's picture (using scratches, irregularities, rotation of hologram, etc...) if the picture is good enough.
Wouldn't this only make sense, to proof that the picture you show was indeed taken by Mice and not by you, so it's not a counterfeited coin you are selling, but indeed a casascius coin?
Yes, Mike will likely do something like this himself and that will be even better. However: he can't do it for the currently existing coins. But these coins are probably going to be the most desirable collectors items. Right now only the person in possession of a coin can do this.
What is the purpose of you doing this?
To enable the public to verify that a specific physical coin has existed at a certain point in time (e.g. April 11th 2013). Since it's unlikely that we already have good counterfeits floating around and therefor all currently existing coins are likely to be authentic, this results in proof of authenticity of the coin at some future date.
Think about a collector getting his hands on one of these rare series 1 coins. Let's say it's the year 2023. If he can prove without a doubt that this coin has existed in April 2013 (which is possible for coins we today "certify the existence" of), he has a much higher likelyhood of actually looking at an authentic coin.