Rory, I find that writing bios for people I never knew turns into a kind of litany that always seems to have the same template - He/she was born (when and where), parents were (links to their profiles). He/she married (link to spouse) and they had (number) of children, (links to children). Then there are paragraphs that state facts in any census, immigration, military service, etc. records, followed by He/she died (where and when) and was buried (where and when). That's "just the facts", but sometimes I can find little extras, like occupation or physical description. I hate that these profiles are so impersonal because I don't know anything to personalize them and imbue their biography with the meaning of their lives.
On the other hand, writing profiles for people I did (or still do) know is the absolute opposite - these people are (obviously) more recent and often still living, so I am careful to deliberately OMIT the sources to protect privacy, while writing all about the nature of who they were. In a way, these seem inappropriate to put on WikiTree because they are essentially lacking in genealogical information while very long on the personal kind of stuff (being careful to ONLY include nice things), so I tend to avoid writing profiles for living people. I can offer two examples of exceptions. I wrote a bio as a tribute to commemorate the birthday of an important person in my life and, when my son was about to be married, I wrote his bio specifically for the benefit of his new bride.