I've noticed that a lot of people started off on LJ making an account just to comment on their friends' posts, not intending to ever write anything of their own.
Why did you first make an LJ account?
To read what my "real-life" friends were posting, and to comment non-anonymously on them.
To read what my online friends or communities were posting, and to comment non-anonymously on them.
To read stuff written on LJ by people I didn't already have any sort of friendship with.
To keep a journal/blog that my "real-life" friends could easily read.
To keep a journal/blog that my online friends could easily read, and/or to participate in communities.
To keep a blog that I wanted people I didn't know at the time to read; to build an audience.
To keep a journal online, though I wasn't expecting anyone to read it at the time.
Because I worked for LJ, and it would've looked bad if I didn't have an account.
None of the above.