Why do i need to import Fetcher from @cloudflare/workers-types

`
import puppeteer from "@cloudflare/puppeteer";
import { Fetcher } from "@cloudflare/workers-types";

interface Env {
    MYBROWSER: Fetcher;
}

export default {
    async fetch(request: Request, env: Env): Promise<Response> {
        const browser = await puppeteer.launch(env.MYBROWSER);
        const page = await browser.newPage();
        await page.goto("https://example.com");
        const metrics = await page.metrics();
        await browser.close();
        return Response.json(metrics);
    },
};

`
import puppeteer from "@cloudflare/puppeteer";
import { Fetcher } from "@cloudflare/workers-types";

interface Env {
    MYBROWSER: Fetcher;
}

export default {
    async fetch(request: Request, env: Env): Promise<Response> {
        const browser = await puppeteer.launch(env.MYBROWSER);
        const page = await browser.newPage();
        await page.goto("https://example.com");
        const metrics = await page.metrics();
        await browser.close();
        return Response.json(metrics);
    },
};