Being vulnerable has two very important implications. Here is the first: when you make a vulnerable game or slam contract you get a bigger score than when you are not vulnerable. Two examples: three notrump making four scores 430 when not vulnerable and 630 when vulnerable; six spades making six scores 980 when not vulnerable and 1430 when vulnerable. This actually doesn’t matter very much at matchpoints, but it matters a lot in team games; in team games, stronger pairs are quite aggressive about bidding thin vulnerable games.
And the second: when you do not make your vulnerable contract (whether a part-score, game or slam), your negative score is a lot worse than if you are not vulnerable. Two examples; down one not doubled is minus 50 when not vulnerable and minus 100 when vulnerable; down three doubled is minus 500 when not vulnerable and minus 800 when vulnerable. This matters a lot at any form of scoring; stronger pairs are more cautious when vulnerable, especially in competitive auctions, and especially when they can expect the opponents to have significant high card strength.
In tournament bridge, you and partner will be “vulnerable” half the time. When you sit North-South, in a session of 24 boards, you will be vulnerable on boards 2, 4, 5, 7, 10, 12, 13, 15, 18, 20, 21, and 23. When you sit East-West, in a session of 24 boards, you will be vulnerable on boards 3, 4, 6, 7, 9, 10, 13, 16, 19, 20, 22, and 23. Neither side will be vulnerable on boards 1, 8, 11, 14, 17, and 24.