Instead of assigning (or hiring etc) your own photographers, stock photography is a set of ready-to-use contents for subscribers who purchased the rights to use them. With digital stock photography, you get to see the final content even before you are granted the rights to use them and these contents can be licensed immediately. The cost for stock photography, especially royalty-free contents are much lower compared to assignment photography. Stock photography saves you model, photographer, location, props, art direction and editing costs.
Royalty-Free is a concept where the user of the content does not have to pay any additional royalties/fees for repeated use of the content after paying the initial licensing fee.
The buyer does not own the copyright of the content but have purchased the rights to use the content legally without additional fees or interruption from the copyright owner provided that the scope of use remains within the End User License Agreement.
These content cannot be re-sold or transferred but it offers a wide range of benefits; mainly lower pricing and convenience.