It helps to group people into common experiences that could help with research. Perhaps members of the same military unit, or those who lived in the same area. Some are for maintenance purposes to allow projects or other users to find and fix them easier (unsourced, or those needing profiles built)
I use personal categories as a way to keep track of profiles that need specific things, like adding all of their siblings, condirming that i have researched to find all spouses, finding a cemetery (or evidence of not being in a cemetery), etc.
I see you have been removing categories related to members of Parliament on some profiles you are managing. This would make it harder for others to locate all members of Parliament for those years. Is there a particular reason why NOT to include those categories? They do not appear to be incorrect?