The mandatory apps will automatically be added to your "My Apps" section and can't be unfavored (the grey star next to the app is disabled).
Depending on the way your administrator configured the App Center, there might be a section "Recommended Apps".
These recommendations can be personalized based on your profile information, see Recommended Apps.
For example, there could be recommendations based on your language, your division or other criteria. This helps you selecting important apps, but they'll only show up in your "My Apps" section if you favorite them manually.
If you don't see any recommended apps, this feature was probably not configured by your admin.
Apps can be assigned to one or more categories. This can be useful if you have a large amount of Apps.Within a category you can have nested categories.
In the image on the left the normal categories are "Atlassian Software" and "Marketing", whereas "Marketing" contains a sub category "Social Media".