The advantages of insulation are mostly energetical: lower heating costs, sound insulation and living comfort. You are able to benefit from insulation in every part of your house, not only under the roof.
The costs for the insulation are individual and can only be named after the check of the overall situation by a professional/ master roofer.
A belated installation is always possible. If you need an insulation over the rafters, under or in-between depends n the rooftop and the execution. The circumstances will be valued on site and then the right material will be chosen.
A rooftop renovation is necessary if smaller or bigger damage influences the use and even bigger damage will result of that. If there is heat loss (if energy cost expenses should be saved/ environmental protection/ conservation of resources) or with bigger construction work at the rooftop (installation of a dormer for example) there has to be a professional revision of the insulation.
If you are applying for Kfz-funds you have to renovate as well.
That depends individually on the condition of the rooftop and the kind of measures that have to be done.