Why blog when what you really want is to be a fiction writer?
That is a valid question.
After all, our energy is not infinite, nor is our attention.
Why waste creative time on something other than your goal?
To put it simply, blogging accustoms one to sharing. Getting used to writing publically takes time. Daily dedication to blogging need not take a lot of time.
A blog gives space for reflection. Regardless whether or not it is read, the writer benefits. Blogging is another craft. Development in blogging will spur creative development.
The two practices need not be mutually exclusive.