Textures & patterns help people who are looking to add creative designs to their projects. It will be so good to have textures in free versions as it will save so much time. Most designers like to get just credit of their work as an appreciation for their hardwork, so do credit them back if you happen to use any of these textures. Most of these are free for commercial uses like web designs, brochures, poster, flyers etc.
Rainbows are something that can be related to happiness, love, friendship, peace etc with 7 colors representing it. Thus using these rainbow textures can add freshness in your design.
20 collection of Rainbows Source