One thing pretty much every successful artist has is clarity of vision. The work fits together, it's a coherent body of work. Whether the artist is truly visible in the body of work or not, the work creates the illusion that the artist can be discerned. One believes that one recognizes the work collectively as all work by so-and-so. Successful artists are not, in fact, just drunkenly welding together random piles of crap, they are welding together piles of crap in a very very purposeful way.
Whether the artist meditates naked in the rain for 24 hours before picking up a chisel, or instead simply drives forward welding, painting, cutting, toward some indefinable goal barely felt, the result is the same. The result is the product of a clear vision. Can the vision be put into words? Is the vision clear in advance, or only when the work is complete? These are not questions that particularly matter.
The clarity of vision creates the coherent body of work. The body of work is arguably the thing that matters, each piece being necessarily similar to something else we have seen. In photography this is clearly a big problem, since so many photographs already exist. In other media, comparisons can still be made: "This sculpture feels a lot like so and so, but bigger, and I sense a strong influence from what's his name" and so on. The body of work is really where the artist speaks clearly. The body of work expresses the artist's ideas, the process of creating that body of work drives forward and refines the artist's ideas. If every day is a brand new day, and you're shooting something new each day, you're not developing anything. You're not going anywhere.
The body of work is the artist's brand. Fine art is as much a branding effort as it is an artistic one, after all. Without a body of work, there's no market. A tastemaker isn't going to be happy with a single work. Why invest time and effort into an artist, unless there will be opportunity to sell other works? These other works need to be identifiably by the same artist, identifiably evolutionary work connected with previous work. Nobody wants to sell or buy a sculpture by a hot new artist only to learn that it was not a bold new work by a rising thought leader, but rather was a one hit wonder.
Commercial artists need the body of work for essentially the same reason. Nobody wants to hire a wedding photographer who's going to do something random and new every time. They want to hire a wedding photographer who is going to produce work pretty much like what's in the portfolio. Ditto fashion, and every other commercial venue. You're hired on the basis of the portfolio, which needs to be a coherent body of work which expresses something pretty specific. It is precisely that specific thing expressed which you are selling, as a commercial artist.
The only photographers with the luxury of not creating a body of work are the amateurs, and they would be well served by nonetheless working at a body of work. While there are no dollars on the line, the satisfaction of following a seam of gold to its conclusion, to developing a set of ideas and pulling together a coherent "collage" of work around them must surely have value. In particular, it's pretty much the only way you're going to make anything genuinely new.