"Run to your houses, fall upon your knees, pray to the gods to intermit the plague, that needs must light on this ingratitude"
"Men at some time are masters of their fates.The fault, dear Brutus, is not in our stars, but in ourselves, that we are underlings".
"Take this paper, and look you lay it in the Praetor’s chair,Where Brutus may but find it; and throw this in at his window;"
The significance of this is that they believe in order to avoid suffering under Caesar's leadership they need to pray that their ingratitude they have showed towards Pompey will be forgiven showing they much preferred Pompey over Caesar which helps to show the significance of the everyday citizens opinions of their leaders.
The significance of this is that they think that they are in control of their future and what will happen to them that it is not set in stone and they will be able to change where they stand under Caesar.
This is significant because these letters are gonna help further convince Brutus to overthrow Caesar by making it appear that even the people want Brutus to be in power more than Caesar.