It's an important question. If you are going to put time and effort into creating a Blog (and putting enough thought into it to make it interesting) it is going to take time. So why would you choose to use your time on a blog versus doing something else?
Bold as brass on this one. If you do not have a purpose for your blog you are wasting your time. Blog's are not just for 'launch day and a flurry of the few days after', blog's are a long term commitment to provide content regularly and consistently... TO DO SOMETHING!
To do what?? ... well... that's the question isn't it?
Why? ... because if you do not have a purpose... if you do not have a reason... if the WHY isn't big enough you will stop doing it. If the passion isn't there people won't read it... you won't share it... it won't be shared... it won't be successful.
PLEASE ... don't blog if you are doing it because someone said... "you must start a blog, everyone's doing it"... if you are thinking of starting a blog... seek help immediately... from a blogger (not a doctor).