"Why should billions of our hard-earned money be spent to send back images of worlds covered with...lots of ordinary looking rocks?

Because the rocks on Earth are only one entity.
We need to spend our money searching and analyzing rocks on all planets so we can better understand and help our Earth and humankind. I would ask this person if they were sick would they want to be treated from knowledge gained from one other person with the same symptoms or want to be treated from the knowledge gained from several different types of people. The knowledge gained from studying several different people is what has helped humandkind treat that one person
The more knowledge gained about rocks and conditions on other planets will help us understand Earth and possible help Earth and humankind in the future. This is why we need to invest our money in other worlds.
What if we found an Earth-like planet in an other solar system orbiting a star just like the Sun, but the planet had died for some reason. Would you want to know why? Maybe analyzing those rocks would help.

SteveS