It's used to show variants of glyphs. @Vitox wrote a good answer already. I'm supplementing it with examples of Chinese characters. All these characters are the same code point (U+85AB) but notice they are slightly different.

E0101, E0102, E0103 are the variation selector 18, 19 and 20.

If you type U+85AB and then the variation selector in a supported text editor, you can type different variants of the same Chinese character.

source: https://youtu.be/cNv2BrmP9SY?t=30