Poker is a mental game: cards play an important role but it's often more important what the other people think about your cards. If you can make them think, by your playing style and other actions at the table, that you have good cards and they believe you, then you can trick them into playing while you have really bad cards in reality.
If your image at the table is that you are playing rather conservatively and all of a sudden you're making a big bet then that can be suspicious: what is going on here? Is it a bluff or do you really have good cards? You will leave the others guessing. It's not always about bluffing because it can also work the other way: if you have good cards you may want everyone to think that your cards are bad, if that suits the situation.