Originalism isn't just one theory, it's actually a family of theories.
Almost all originalists agree on two ideas. The first idea is the Fixation Thesis. The Fixation Thesis claims that the original public meaning of the constitutional text is fixed at the time each provision is framed and ratified. Now why is that the case? The Constitution uses words and phrases, and it combines them into clauses, into meaningful units that are structured by grammar and punctuation. If we want to know what that text means, we want to know what the words meant at the time the constitutional text was written.
For example, Article Four of the Constitution uses the phrase domestic violence. The contemporary meaning of that phrase is limited to violence within a family, elder abuse, child abuse, spousal abuse, but that's not what article four is talking about. In Article Four, the phrase domestic violence refers to riots, insurrections, and rebellions within a state. It would just be silly to interpret the constitutional text using the contemporary meaning of the words and phrases. We ought to use the meanings that existed at the time.
The second idea, upon which originalists agree, is the Constraint Principle. This is the idea that the constitutional text ought to be followed, that it should be binding. Now why would we think that? There are many reasons.
Idea number one, the Constraint Principle is necessary for the rule of law. Without constraint, the Supreme Court is free to adopt constructions of the Constitution that have the same effect as amendments. The Supreme Court decides issues on a case-by-case basis. When you have the power to establish fundamental law, on a case-by-case basis, and you're not required to be consistent, the Supreme Court can overrule its own precedence, then you have tyranny.
There's another reason why the Constraint Principle is important to the rule of law and that is that in the alternative, the Supreme Court is likely to become politicized, and of course we've seen that in recent confirmation hearings. The Senate and Presidents have come to realize that it's possible to use the Supreme Court as a super legislature, and once you have that realization, then the nomination and confirmation process becomes politicized, and a politicized process leads to a politicized Court. And the politicization of the Supreme Court is antithetical to the rule of law.
There's another reason why the Constraint Principle is important, and that's democratic legitimacy. Now, the Constitution is imperfectly democratic for familiar reasons. Critics of originalism love to point this out, and they're right. The original Constitution was not ratified by all the people, the franchise was limited. Basically women and persons of color and Native Americans were excluded from that process, but by the standards of the time, the process of constitutional ratification was intensely democratic. And if you want to compare the process of constitutional ratification to Constitution making by a committee of five, it's clear which of the two processes wins on the criteria of democratic legitimacy.