Democratic elections are meant to do two things. They are supposed to be used to select the people who will represent us in government. They are also supposed to be a way for us to make our policy preferences known to the government. Political parties can help allow both of these to happen, but they do not necessarily do so.
In elections, we are supposed to be conveying our desires for or against certain policies that the government might adopt. Parties can help us do this. We typically know what a given party stands for in terms of policies. Therefore, we know that a vote for (for example) the Republican Party in the US is probably a vote for lower taxes and policies that support more traditional social values. Parties thus make it easier for us to know what we are voting for or against.
However, parties do not always make things clear. In the US, we do not have what is called “responsible party government.” In parliamentary systems, one party (or a coalition) is always in power and has the ability to pass its agenda. In our system, we can (as we do now) have divided government. This makes it difficult for us to know which party is to be blamed or credited for a given state of affairs. This makes it harder for us to know which party to vote for or against. It makes it harder for us to use our votes to make our feelings known.