I had long considered writing my own life story but it only came to fruition after joining this group.
Feeling obligated to produce some story or event each month, to be read out to  the group for their amusement or observations, positive of course, ensured progress was made and a book finally transpired.
Writing your life story can be for a number of reasons.  It will usually be for your family, friends and descendants, but it can be just for you.  A chance to reflect on your life, and this can be so therapeutic.  But always,  and usually,  it will be satisfying and fun.