Firstly people will be afraid to support their opinion via voting in many cases because they will be concerned not so much with their friends, but with their employers and colleagues. In Rome only citizens could vote, and sure not all citizens were rich but they were generally speaking not the bottom rungs of society and would be less concerned about their job than many of the poorest citizens in today's society.
It isn't just intimidation it is also a method by which buying votes directly is disincentivized.
Sure technically I can claim to sell my vote now, but the person buying it is taking it on faith I am actually selling it, which means nobody tries to actually buy votes directly. They will attempt to buy votes by say, bribing people in certain positions to endorse them, but they won't try and do so directly.