The short answer — you do. I have to assume you have developed your interest in photography over a good period of time. Your interest might have started with a photograph you saw at a local art fair. That's how it started with me, and yes, I do buy other photographers' work — it isn't a one-way street to me. I think the first one was a black and white image; it is still in the original mat. I looked at it for a while. I like it, but not enough to frame it. A couple of years later I bought my first signed, limited edition print, beautifully framed by the way. A sepia study, "Heart of Saturday Night," hangs here in my office. After that came a magnificent artist's proof, a dye transfer print by American-Israeli photographer Neil Folberg. It is a photograph of the inside of a synagogue — the type of image that reveals secrets over the years. I just love it.
The point, of course, is that a photograph becomes collectible to you when the image carries intention, when it is something you will gaze upon and appreciate over years, not just the first hour after you purchase it. Of course the photographer's vision, the authenticity, the archival print, Certificate of Authenticity (COA), and presentation are important, but those things should be a given before you select a piece to grace your space.
I'm known for embracing the statement "I don't need to know how the sausage is made." Yes, I am a photographer, and when I look at another artist's print I will probably appreciate the process behind it more than some other people. When it comes to the final print, is that relevant? For me, the impact of the photograph — its implied narrative, little surprises, sometimes mystery — is what makes it appealing. That's what compels me to collect and hang it on my walls.