Some variables might be...
If it's said to you on first written encounter on this site (or any other one) then they probably want to go slow and be friends first.
If it's said after more written "back and forths" they probably aren't interested beyond friendship.
Add a phone chat or two to multiple "back and forths" and still 'friendship only'...accept it as a 'friends only' and make up your mind whether you can be friends only with this person.
If it's said after a F2F then it may (or may not!!) be that they aren't sexually attracted but would still entertain a friendship.
Or....they may just not know how to tell you that they aren't "into" you at all and figure that suggesting friendship is a kinder blow-off than "get lost"!!!
Or....they may be like I am with a status on this site of "seeking friendship only" and figure a reminder may be in order!
Or....etc, etc., etc.
I used to have that in my profile start out as Friends, blah, blah, blah, but then I realized I do sound like Kenny mentioned and do not contact me without Extreme Caution when approaching, haha. I wouldn't call it starting out as friends but dating, dating to see if there is a compatibility, and then move into the friend/the one zone. I mean seriously, if you date and it don't work out, could you still be just "Friends"? Not too often, but maybe. My friends mean the world to me and I can talk to them about anything and starting out as friends would be too Bold. Again, I would call it dating. Don't need more junk email;)
Good luck ya'll.