Why do the four men give up their seats for jem, scout, dill, and reverend sykes? what does this show us?

The men gave up their seats because they were black and Jem, Scout, Dill, and Reverend Sykes were white. This episode is a dramatic illustration of the racial caste system that existed in the town. The black citizens could not enter, let alone sit down in, the main section of the courthouse but were instead consigned to the balcony. And, even when in the balcony, if a white person -- even a child -- chose to enter and had no seat then black citizens were obliged to stand.