The purpose of "reimport" is to get FEA changes into the model without losing any settings to components that already exist. Say that you have added elements to the model, or deleted elements from the model. Then you do a reimport, so that new components are created, or components that have been removed from the model are removed from HyperSizer. The components that were not changed in the FEM should not be changed in HyperSizer.
When you "Remove FEM", you are deleting all of the components. Group data is different, however. Groups are not tied to the FEM. You can have a group with one component, ten components or zero components (in which case, the group will do nothing). So the groups do not get deleted when removing the FEM. When you then import again from scratch, you can assign the newly imported components into these groups.
If you are using the "Create Groups from PCOMPs" option, what this means is that when you do the import after removing the FEM, the old groups are not overwritten, but new groups are created.
There is additional discussion on reimport and remove FEM in the HyperSizer Pro User Manual. Search for the label "Updating the Model in HyperSizer"