It's because such research has no obvious initial use that the public must pay for it; no private enterprise will fund it, and often it will be useless knowledge, but occasionally someone will figure something out that unlocks a whole new understanding of the world.

It's publicly-funded venture capital for ideas.

IIRC even LASER was seen as a novely demonstration of quite an obscure effect…

Gladstone once asked Faraday about the usefulness of electricity, just saying.

Faraday's response: "Why sir, there is every possibility that you will soon be able to tax it!"