Well he probably takes photos to learn more about how the Pokemon look like and also pictures of where he already been too so he can remember them when he goes to other towns. Its his hobby to take photos so he could pretty much remember all the places he has been too.
I think its just a hobby. But it is possible for it to be than just a hobby.., like @JB said. Maybe its just a little promo for a new Pokémon snap later down the road? Lolz.