In the context of your question, you are speaking about Chapter 2 in this impeccable piece of historical fiction set in the time of Christ called The Bronze Bow by Elizabeth George Speare. Even though you ask about Rosh, the main character of the story is actually Daniel bar Jamin. Rosh is the leader of the rebel group hanging out in the mountains (while Daniel, at least at the beginning of the story, is a member of the rebel band). Rosh is all about violence and bloodshed against Romans. He is one of those people who thinks that the Kingdom of God can only come about through defeating the Romans militarily. Knowing this about Rosh, it gives us an insight as to why he might want a slave that comes to be known as “Samson.” Rosh wants to capture this particular slave because he can truly be of use to the rebels and their cause against the Romans.
The wording of your question is interesting because Rosh doesn’t just want “a slave,” instead he wants only “this slave.” Why? Because the slave to be named “Samson” later is not typical. How do we know it is important to Rosh? Look at the exchange between Ebol (another member of the rebel group) and Daniel:
“Easy. No guard to speak of. All we’re to take is one slave.”
“Not a thing but the slave. A black fellow, big as an ox. Rosh spotted him yesterday when they stopped at Merom. Too good to waste on the galleys, Rosh says.”
This exchange tells the reader a lot. Specifically, it tells us that Rosh thinks that Samson is “too good to waste on the galleys.” Too good to kill, in other words. What is more important is what is implied here. If Samson is “big as an ox,” then Rosh most likely wants this “too good” slave to be of use the rebels in regards to physical labor. This is exactly what happens. What we don’t know at this time is that Samson is a mute (in that he doesn’t talk at all).
After Samson is taken from the packtrain we learn about Daniel’s new doubts about Rosh’s choice:
There was power there, all right. Those huge arms could crack the ribs of a man as easily as a child could snap a twig. But the broad face with the livid scar showed no sign of intelligence, only an animal wariness that would mark the time to strike.
Again, it is implied here, but who would Rosh want to have their ribs broken? The Romans, of course. Thus, the true usage of Samson becomes known indirectly. When Samson is finally brought to Rosh, the leader of the rebels is happy at his new recruit (although a bit unnerved that Samson won’t speak at all). The men begin to grumble in worry that Samson might turn on them in the night and kill them if Samson doesn’t understand whose side he’s on. Finally, Rosh hits the nail on the head:
We took him for his muscles, not his tongue. He’ll prove his worth soon enough.
This shows that Rosh is always promising that the final uprising against the Romans is right around the corner. Samson was chosen for power in that skirmish.
In conclusion, Rosh wants this particular slave he names “Samson” because of his strength (hopefully in fighting the Romans). What is interesting is what actually happens is that Daniel ends up getting a confidante for life. Daniel, as the blacksmith of the group, uses his prowess to set Samson free of his chains. This act of mercy and kindness and freedom bonds the two together (or perhaps I should say bonds Samson to Daniel). It takes a while for Daniel to warm up to Samson, but eventually Samson saves Daniel’s life, and Daniel will never forget that act just as Samson will never forget his freedom. I’m afraid it is Rosh, by the end of the book, who has chosen the wrong methods for success. Instead of war, the character of Jesus teaches us in The Bronze Bow that it is mercy and kindness and compassion that are key, not war.