Sorry about stupid question, but what could you do with this account? WHy it's worth something more than usual?
in case you're not familiar, reddit is a massive community, as in one of the largest on the net. So it is a valuable platform for people wanting to let others know about their blog, website, videos, products, whatever. The problem is that generally reddit users aren't too happy when people try to promote their stuf so you need to be somewhat subtle about it. Having a long standing account with lots of karma makes you look more like a legitimate user. I do not think anyone would look at my account and think it would be an account for promotion. So the promotion you post looks genuine.
Getting caught using accounts solely to advertise can carry various issues depending on where you get caught. Getting caught in most places will just result in a ban from that subreddit. Getting caught in some of the more popular areas of the site can lead to terrible public image for your product/service, you're website may be banned from that subreddit, ect... However with an account like this, you are much more credible if anybody ever looks into it.
and in my case, being in the top 1% of the site on karma means it can access other private subreddits (such as /r/top). These places don't have massive audiences (as they are locked from most users), but nobody there is going to suspect you of being an advertisement
And like the abover user said, that E-dick for some people. Some users like having an older account or an account with lots of karma solely for personal reasons. Generally though the value is in what I described above.