I think you're asking
1) What you should keep in mind when the photographer is taking a photo of you
and 2) What the photographer may do better than you can yourself?
Being Prepared for the Photo
You want to make sure that the photo is representative of who you are and that you look your best. Stray hairs, small blemishes and wrinkled clothes are a lot more noticeable in photos so it's a good idea to make sure your hair is done, your shirt is ironed and maybe even apply some makeup if you're having a bad skin day. Basically, look how you want to look on the day of the interview and you should be okay. Overall, it's the photographer's job to make you look good so he/she should be helpful on the day of the shoot with making sure you aren't making strange facial expressions and little things like that.
Benefits of going to a photographer
The photographer is a professional, has experience behind him and if it's customary in Germany to provide a photo with your CV, the photographer will probably know what to do and what employers specifically look for. The biggest thing you'll notice between what a professional photographer will do and what you will achieve with a home photo is lighting. A pro photographer should have the appropriate lighting set up which will genrally give you a better exposed photo and eliminate things like harsh shadows on your face. Those are just a couple points but if you're using this as a headshot for something important like a job application it's a very good idea to go to a professional photographer.
It's quite worth the investment to pay for a professional shoot, as the end product will likely be much better than what you'll produce at home. Also, the one investment for the photograph will give you something you can use for several job applications and you can very likely use it several years down the road if the current job doesn't work out.
I hope this was helpful, and good luck with the job application.