Because photos sell properties. My wife went and took about 30 shots of a $500K property that had been on the market for over a year. Within 3 weeks they had it under contract for $495K. The buyers said they had skimmed over the property a time or two in the past but when it popped up on their latest search they actually thought it was a different property because the picture were so much better. Shots from the realtors iPhone just doesn't cut it anymore... You want it sold, you hire someone to take the photos.

That's like asking why computer programmers need to know how to use a keyboard to input text.

The most basic element of a sale is "what am I buying?". If a realtor cannot sufficiently answer that question through photographs and detailed descriptions, then said realtor has failed on a most basic level.

At the very least, a realtor who lacks the ability to take a good photograph ought to hire the services of someone who does (a.k.a. a photographer).