A Boring Dystopia

A candidate might have hundreds of policies. Often they claim to support certain policies because they know those policies are popular, but don't have any real belief or intention to follow up on those policies.

Political parties are effectively a "brand" so that people who don't have the time or energy to read hundreds of policies and to ensure that the candidate claiming them is trustworthy and believes what they're claiming. Parties often kick out members who no longer align with their views. A voter can vote for a certain party's representative and know that what they're getting aligns with that party's general stance.

Left and right is a useful way to represent where various parties fall on a specific spectrum of beliefs. In reality there are multiple axes that can represent economic freedom, alignment with Christian religious beliefs, the strength of a social safety net, isolationist vs. interventionist foreign policy, etc.

But, "left" and "right" came from where groups of politicians stood in the early days of the French republic. They tended to group up with other like-minded people so even if they disagreed completely on certain things, they had generally the same world view. So, "left" and "right" is a good way to simplify things to know if in general a politician shares a certain set of beliefs.