In that case, the requirement to show up at the polling station seems like a pointless infringement on personal liberty.
Not pointless at all: it turns most of the cost of voting into a sunk cost, thus making it much more likely to be rational for any one citizen to cast a vote.
Today is the midterm elections in the United States, and I am not voting.
For the vast majority of elections, voting is irrational, because the individual's vote is proportionately very small. This means it cannot have an effect on the outcome.
There are, however, conditions which can lead to voting becoming rational, and these are: