@lindhe : Adding to Torstens's reply: FMI then comes into play when you want to exchange, co-simulate or deploy simulations models between different tools and simulations platforms.
The Prostep Smart Systems Engneering project has published a simple video on the benefit of FMI: https://youtu.be/A-3iIot0fO8.
Basic FMI papers/slides: FMI 1.0: The Functional Mock-up Interface for Tool independent Exchange of Simulation Models (Slides: PDF, PPT)https://svn.modelica.org/fmi/branches/public/docs/Modelica2011/The_Functional_Mockup_Interface.pdf , https://svn.modelica.org/fmi/branches/public/docs/Modelica2011/The_Functional_Mockup_Interface_paper.pdf
FMI 2.0: https://svn.modelica.org/fmi/branches/public/docs/Modelica2012/ecp12076173_BlochwitzOtter.pdf
A Tutorial can be found here: Gensim Workshop on Modelica and FMI https://github.com/ibpsa/project1/wiki/2016-10-24-gensim-agenda