Some YouTubers like to vlog fun experiences in their life to show to their subscribers. In example, going on a cruise, taking a vacation with their friends, going to a music festival, meeting an idol, or going to an award show could all be made into vlogs. Although, some people like to vlog in a more creative way.
The ones who tend to vlog creatively pick subjects in which their opinion can be portrayed to the world about a certain subject. For example, some YouTubers love to vlog in the form of rant videos, sketches, or music videos to entertain their subscribers while presenting a subject or trend they are familiar with.
A lot of vloggers also like to simply sit in front of the camera and talk about world wide issues or about what is going on in their life. I believe that the majority of people like to watch these types of vlog videos because they could be a spark of inspiration or could be easy to relate to. A YouTuber speaking about his/her personal experience with bullying could be a strong inspiration for those struggling with bullying while watching the vlog.

Most famous YouTubers like Bethany Mota, ThatcherJoe, Zoella, MayBaby, Tanya Burr, Tyler Oakley, Tanya Burr, Jennxpenn, and more, have 2 separate channels on YouTube. One channel they would call their 'main channel' and the other would be strictly for vlog videos. Therefore, they normally have a set schedule for posting on their main channel (like a new video every Tuesday and Sunday) and they have their vlog channel for when they want to post on certain occasions.
In my opinion, vlogs are very entertaining and I tend to watch all sorts of different vlogs that catch my eye.
Resources: http://www.urbandictionary.com/define.php?term=vlogger
http://www.adweek.com/socialtimes/top-youtube-vloggers/11285